IBM zSystems: Make stack_tie to work with hard frame pointer

With this patch a scheduling barrier is created to prevent the insn
setting up the frame-pointer and instructions which save GPRs to the
stack to be swapped.  Otherwise broken CFI information would be
generated since the stack save insns would use a base register which
is not currently declared as holding the CFA.

Without -mpreserve-args this did not happen because the store multiple
we used for saving the GPRs would also cover the frame-pointer
register and therefore creates a dependency on the frame-pointer
hardreg. However, with this patch the stack_tie is emitted regardless
of -mpreserve-args since this in general appears to be the safer
approach.

	* config/s390/s390.cc (save_gprs): Use gen_frame_mem.
	(restore_gprs): Likewise.
	(s390_emit_stack_tie): Make the stack_tie to be dependent on the
	frame pointer if a frame-pointer is used.
	(s390_emit_prologue): Emit stack_tie when frame-pointer is needed.
	* config/s390/s390.md (stack_tie): Add a register operand and
	rename to ...
	(@stack_tie<mode>): ... this.

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