s390: Represent FP without RA saved in SFrame

If an architecture uses both SFrame RA and FP tracking SFrame assumes
that the RA offset is the 2nd offset and the FP offset is the 3rd offset
following a SFrame FRE.  An architecture does not necessarily need to
save both on the stack (or in register) at the same time or even at all.
SFrame cannot represent FP without RA saved on stack (or in a register),
since it cannot distinguish whether the 2nd offset is the RA or FP
offset.

For s390x use an invalid SFrame RA offset from CFA value of zero as
padding to represent the FP being saved when the RA is not saved.  This
aligns with the existing invalid SFrame fixed RA offset from CFA value
of zero.  In a stack tracer this then also naturally falls into place,
as it can skip restoring the RA in the topmost frame, if both the fixed
RA offset (from SFrame header) and the RA offset (from FDE) are zero,
without any need to test architecture-specific flags.

include/
	* sframe.h (SFRAME_FRE_RA_OFFSET_INVALID): New define.  Used as
	padding offset.
	* sframe-api.h (sframe_fre_get_ra_offset): Add comment that for
	s390x an offset value of SFRAME_FRE_RA_OFFSET_INVALID indicates
	that the RA is not saved.

gas/
	* gen-sframe.c (get_fre_num_offsets): For s390x account padding
	RA offset, if FP without RA saved.
	(sframe_get_fre_offset_size): Likewise.
	(output_sframe_row_entry): For s390x write a padding RA offset,
	if FP without RA needs to be represented.
	(sframe_do_fde): Enable FP without RA saved to be represented
	on s390x.

libsframe/
	* sframe.c (sframe_fre_get_ra_offset): Add comment that for
	s390x an offset value of SFRAME_FRE_RA_OFFSET_INVALID indicates
	that the RA is not saved.
	* sframe-dump.c (dump_sframe_func_with_fres): Treat invalid
	RA offsets as if they were undefined.  Display them as "U"
	to distinguish them.
	* doc/sframe-spec.texi (s390x): Document s390x-specific use of
	SFRAME_FRE_RA_OFFSET_INVALID to represent FP without RA saved.

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Rename s390x-specific tests.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-err-1.s: Rename
	to ...
	* cfi-sframe/cfi-sframe-s390x-fpra-offset-err-1.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-2.s: This.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-offset-2.d: Likewise.
	Update test verification pattern accordingly.
	* cfi-sframe/cfi-sframe-s390x-fpra-register-err-1.s: Rename
	to ...
	* cfi-sframe/cfi-sframe-s390x-fpra-register-err-1.d: Likewise.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-2.s: This.
	* gas/cfi-sframe/cfi-sframe-s390x-fpra-register-2.d: Likewise.
	Update test verification pattern accordingly.

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

include/sframe-api.h

@@ -224,6 +224,8 @@ sframe_fre_get_fp_offset (sframe_decoder_ctx *dctx,
 
 /* Get the RA offset from the FRE.  If the offset is invalid, sets errp.
 
+   For s390x an RA offset value of SFRAME_FRE_RA_OFFSET_INVALID indicates
+   that the RA is not saved, which is only valid in the topmost frame.
    For s390x the offset may be an encoded register number, indicated by
    LSB set to one, which is only valid in the topmost frame.  */
 extern int32_t