[SFrame-V3] gas: sframe: allow SFrame FDEs with 0 FREs for signal frames

[SFrame-V3] gas: sframe: allow SFrame FDEs with 0 FREs for signal frames

Update the SFrame generation logic in GAS to emit Function Descriptor Entries
(FDEs) for signal frames even when no Frame Row Entries (FREs) could be
generated.

Previously, create_sframe_all () would discard any FDE that failed
translation or resulted in zero FREs. However, for signal frames (marked
with .cfi_signal_frame), preserving the FDE may be valuable even without
stack offsets. The presence of the SFrame FDE with the 'Signal'
attribute may allow stack tracers to identify the frame as a signal
trampoline and potentially apply fallback handling, rather than treating
the PC range as having no stack trace info at all.

The patch modifies create_sframe_all () to detect translation errors for
signal frames, effectively allowing the generation of an empty FDE (0 FREs)
marked with the 'S' attribute.

TBD:
 - But the warnings are already issued, we may emit SFrame FDE with no
   FREs.  Somewhat misleading.

gas/
	* gen-sframe.c (sframe_fde_free): Add NULL check for safety.
	(create_sframe_all): Allow FDEs for signal frames even if translation
	encountered errors or produced no FREs.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.d: New test ensuring
	signal frame FDE is emitted with 0 FREs.
	* gas/cfi-sframe/cfi-sframe-x86_64-signal-1.s: New test.