gas: sframe: Represent .cfi_undefined RA as FRE without offsets

gas: sframe: Represent .cfi_undefined RA as FRE without offsets

In DWARF CFI an "undefined" register rule for the return address (RA)
register indicates that there is no return address and the stack trace
is complete.

Represent DW_CFA_undefined as SFrame FRE without any offsets, so that a
stack tracer implementation can use this as indication that an outermost
frame has been reached and the stack trace is complete.

This representation is backward compatible, as existing stack tracers
should already deal with the case, that an SFrame FRE a so far invalid
offset count of zero and stop the trace.

include/
	* sframe.h (SFRAME_V2_FRE_RA_UNDEFINED_P): New macro to test
	FRE info word for RA undefined (FRE without any offsets).

binutils/
	* NEWS: Mention SFrame can represent an undefined RA as FRE
	without	any offsets.

gas/
	* gen-sframe.h (struct sframe_row_entry): Add ra_undefined_p
	flag.
	* gen-sframe.c (sframe_row_entry_new): Initialize ra_undefined_p
	flag to not set.
	(sframe_row_entry_initialize): Treat ra_undefined_p flag as
	sticky.
	(sframe_fre_set_ra_track): Reset ra_undefined_p flag.
	(sframe_xlate_do_restore): Reset ra_undefined_p flag to saved
	state.
	(sframe_xlate_do_same_value): Reset ra_undefined_p flag.
	(sframe_xlate_do_cfi_undefined): For RA set ra_undefined_p flag.
	(output_sframe_row_entry): Represent RA undefined as SFrame FRE
	without any offsets and FRE info word fields zeroed.
	* NEWS: Mention assembler represents .cfi_undefined RA in SFrame
	as FRE without any offsets.

libsframe/
	* doc/sframe-spec.texi (Changes from Version 1 to Version 2):
	Mention that a SFrame FRE without any offsets flag indicates an
	outermost frame with an undefined RA.
	(fre_offset_count): Document that a FRE offset count of zero
	indicates an outermost frame with an undefined RA.
	* sframe.c (sframe_get_fre_ra_undefined_p): Use macro
	SFRAME_V2_FRE_RA_UNDEFINED_P.
	(sframe_fre_get_fp_offset, sframe_fre_get_ra_offset): Do not
	return fixed FP/RA offset if RA undefined.
	* sframe-dump.c (dump_sframe_func_with_fres): Show FRE without
	any offsets as "RA undefined".

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Run tests for .cfi_undefined RA
	on AArch64, s390x, and x86-64.
	* gas/cfi-sframe/cfi-sframe-aarch64-ra-undefined-1.d: Add test
	for .cfi_undefined RA on AArch64.
	* gas/cfi-sframe/cfi-sframe-aarch64-ra-undefined-1.s: Likewise.
	* as/cfi-sframe/cfi-sframe-s390x-ra-undefined-1.d: Add test
	for .cfi_undefined RA on s390x.
	* gas/cfi-sframe/cfi-sframe-s390x-ra-undefined-1.s: Likewise.
	* gas/cfi-sframe/cfi-sframe-x86_64-ra-undefined-1.d: Add test
	for .cfi_undefined RA on x86-64.
	* gas/cfi-sframe/cfi-sframe-x86_64-ra-undefined-1.s: Likewise.

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