s390: Initial support to generate .sframe from CFI directives in assembler

This introduces initial support to generate .sframe from CFI directives
in assembler on s390 64-bit (s390x).  Due to SFrame V2 format
limitations it has the following limitations, some of them getting
addressed by subsequent patches, which cause generation of SFrame FDE
to be skipped:

- SFrame FP/RA tracking only supports register contents being saved on
  the stack (i.e. .cfi_offset).  It does not support FP/RA register
  contents being saved in other registers (i.e. .cfi_register).  GCC on
  s390x can be observed to save the FP/RA register contents in floating-
  point registers, but only in leaf functions.
  This issue is detailed further and resolved in the subsequent commit
  "s390: Represent FP/RA saved in register in SFrame".

- SFrame FP/RA tracking cannot represent FP without RA saved.  This is
  because the format assumes SFrame FDE offset2 to be the RA offset, if
  there are two offsets, and offset3 to be the FP offset, if there are
  three offsets.  There is no mean to distinguish whether offset2 is the
  RA or FP offset, if there are only two offsets.
  This issue is detailed further and resolved in the subsequent commit
  "s390: Represent FP without RA saved in SFrame".

- SFrame assumes a dedicated FP register number.  The s390x ELF ABI [1]
  does only designate register 11 as preferred FP register number.  In
  general GCC and Clang on s390x use register 11 as frame pointer.
  GCC on s390x can be observed to use register 14 as frame pointer in
  the stack clash protector in the function prologue.
  glibc on s390x contains hand-written assembler code that uses
  register 12 as frame pointer.

This s390x support is largely based on the AArch64 support from commit
b52c4ee46657 ("gas: generate .sframe from CFI directives").

The SFrame ABI/arch identifier SFRAME_ABI_S390X_ENDIAN_BIG is introduced
for s390x and added to the SFrame format specification.

The s390x ELF ABI [1] specifies the following C calling conventions for
s390x architecture:
- Register 15 is the stack pointer (SP).
- Register 14 contains the return address (RA) at function entry.
- There is no dedicated frame pointer register.  Register 11 is the
  preferred frame pointer (FP). [2]  GCC and Clang in general use
  register 11 as frame pointer.
- The CFA is defined as SP at call site +160. [3]  The SP at call site
  can therefore be derived from the CFA using a SP value offset from CFA
  of -160.

The s390x ELF ABI [1] does not assign any standard save slot to each
register in the register save area of a stack frame.  Neither the
return address (RA, r14) nor preferred frame pointer (FP, r11)
necessarily need to be saved.  Therefore SFrame RA and FP tracking is
used.

Support for SFrame on s390 is only enabled for the 64-bit s390x ELF ABI
(z/Architecture with 64-bit addressing mode).  It is disabled for the
32-bit s390 ELF ABI (ESA/390 or z/Architecture with 32-bit addressing
mode).

s390x-specific SFrame assembler and linker tests are added, including
error tests for use of a non-preferred frame pointer (FP) register and
specification of a non-default return address (RA) register.

[1]: s390x ELF ABI, https://github.com/IBM/s390x-abi/releases
[2]: s390x ELF ABI, commit f00421825979 ("Add information about the frame
     pointer register"),
     https://github.com/IBM/s390x-abi/commit/f00421825979
[3]: s390x ELF ABI, commit 4e38ad9c8a88 ("Document the CFA"),
     https://github.com/IBM/s390x-abi/commit/4e38ad9c8a88

include/
	* sframe.h: Add reference to s390x architecture in comments.
	(SFRAME_ABI_S390X_ENDIAN_BIG): Define SFrame ABI/arch identifier
	for s390x.
	(SFRAME_S390X_SP_VAL_OFFSET): Define s390x-specific SP value
	offset from CFA.

libsframe/
	* sframe.c (need_swapping): Add SFRAME_ABI_S390X_ENDIAN_BIG.
	* doc/sframe-spec.texi (SFRAME_ABI_S390X_ENDIAN_BIG, s390x,
	SFRAME_S390X_SP_VAL_OFFSET): Document SFrame ABI/arch identifier
	for s390x, add references to s390x architecture, and document
	s390x-specifics, such as the SP value offset from CFA of -160.

gas/
	* config/tc-s390.h: s390x support to generate .sframe from CFI
	directives in assembler.
	(support_sframe_p): Define.
	(SFRAME_CFA_SP_REG, SFRAME_CFA_FP_REG, SFRAME_CFA_RA_REG):
	Define.
	(sframe_ra_tracking_p): Define.
	(sframe_cfa_ra_offset): Define.
	(sframe_get_abi_arch): Define.
	* config/tc-s390.c: s390x support to generate .sframe from CFI
	directives in assembler.
	(s390_sframe_cfa_sp_reg, s390_sframe_cfa_fp_reg,
	s390_sframe_cfa_ra_reg): New.  Initialize to DWARF register
	numbers of stack pointer (SP, r15), preferred frame pointer
	(FP, r11), and return address (RA, r14) registers.
	(s390_support_sframe_p): New function.  Return true if s390x.
	(s390_sframe_ra_tracking_p): New function.  Return true.
	(s390_sframe_cfa_ra_offset): New function.  Return
	SFRAME_CFA_FIXED_RA_INVALID.
	(s390_sframe_get_abi_arch): New function.  Return
	SFRAME_ABI_S390X_ENDIAN_BIG if s390x, otherwise zero.
	* gen-sframe.c: Add reference to s390x architecture in comments.
	(sframe_xlate_do_val_offset): Add support for s390x-specific
	SFRAME_S390X_SP_VAL_OFFSET.
	* NEWS: Add news entry.

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Enable common SFrame tests for
	s390x.  Add s390x-specific SFrame (error) tests.
	* gas/cfi-sframe/cfi-sframe-s390x-1.d: New s390x-specific SFrame
	test.
	* gas/cfi-sframe/cfi-sframe-s390x-1.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-2.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-2.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-err-1.d: New s390x-specific
	SFrame error test that uses a non-default frame-pointer register
	as CFA base register.
	* gas/cfi-sframe/cfi-sframe-s390x-err-1.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-err-2.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-err-2.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-err-3.d: New s390x-specific
	SFrame error test that uses a non-default return address
	register.
	* gas/cfi-sframe/cfi-sframe-s390x-err-3.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-1.d: New s390x-
	specific SFrame test that saves RA and FP individually on the
	stack.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-1.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-err-1.d: New
	s390x-specific SFrame error test that saves FP and RA
	individually, to trigger FP without RA saved.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-err-1.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-1.d: New
	s390x-specific SFrame error test that saves FP and RA
	individually in registers.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-1.s:
	Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-2.d: New
	s390x-specific SFrame error test that saves RA and FP
	individually in registers.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-err-2.s:
	Likewise.

ld/testsuite/
	* ld-s390/s390.exp: Add simple SFrame test.
	* ld-s390/sframe-simple-1.d: New simple SFrame test.
	* ld-s390/sframe-bar.s: Likewise.
	* ld-s390/sframe-foo.s: Likewise.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>

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