include: sframe: doc: define new flag SFRAME_F_FDE_FUNC_START_PCREL

Add a new flag SFRAME_F_FDE_FUNC_START_PCREL to SFrame stack trace
format.  If set, this flag indicates that the function start address
field (sfde_func_start_address) is the offset to the function start
address from the SFrame FDE function start address field itself.

Such an encoding is friendlier to the exisitng PC-REL relocations
available in the ABIs supported in SFrame: AMD64 (R_X86_64_PC32) and
AArch64 (R_AARCH64_PREL32).  In subsequent patches, we will make the
implementation in gas and ld to both:
  - emit the values in the same (above-mentioned) encoding uniformly.
  - set the flag SFRAME_F_FDE_FUNC_START_PCREL in the SFrame header
    for consumers to be able to distinguish.

Define SFRAME_V2_F_ALL_FLAGS in sframe.h to help keep the implementation
less error-prone by keeping a set of all defined flags at a central
place.  Adjust the check in sframe_header_sanity_check_p () to use the
SFRAME_V2_F_ALL_FLAGS instead.

Add documentation for SFRAME_F_FDE_FUNC_START_PCREL.  Update the
documentation about the encoding of the sfde_func_start_address field.

Also, update the section "Changes from Version 1 to Version 2" to
include the specification of the new flag SFRAME_F_FDE_FUNC_START_PCREL
as an erratum to the SFrame Version 2 specification.

include/
        * sframe.h (SFRAME_F_FDE_FUNC_START_PCREL): New definition.
        (SFRAME_V2_F_ALL_FLAGS): Likewise.
libsframe/
	* sframe-dump.c (dump_sframe_header_flags): Update to include
	the new flag SFRAME_F_FDE_FUNC_START_PCREL.
	* sframe.c (sframe_header_sanity_check_p): Use
	SFRAME_V2_F_ALL_FLAGS.
libsframe/doc/
	* sframe-spec.texi: Add details about the new flag.  Also update
	the defails about the sfde_func_start_address encoding.

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