[SFrame-V3] gas: sframe: testsuite: handle .cfi_offset for RA

[SFrame-V3] gas: sframe: testsuite: handle .cfi_offset for RA

With the introduction of flex FDE type, handling .cfi_offset for
RA needs adjustment.

On architectures like x86_64, the return address (RA) is typically saved
at a fixed offset from the CFA. Previous versions of the SFrame format
assumed this fixed offset was invariant for the entire function on such
architectures. Consequently, GAS would warn and suppress SFrame
generation if it encountered a .cfi_offset directive for the RA
register that deviated from this fixed default.

SFrame V3 introduces "Flex FDEs" which allows tracking the RA location
explicitly even on architectures where it is usually fixed.

This patch updates sframe_xlate_do_offset () to leverage Flex FDEs. When
processing a .cfi_offset for the RA register:
  - The check for non-representable RA offsets is relaxed. If the ABI
    supports Flex FDEs (SFrame V3), GAS proceeds instead of issuing a
    warning.
  - For ABIs without explicit RA tracking (like AMD64), if the RA
    offset differs from the default fixed offset, the FDE is marked as a
    Flex FDE, and the new stack location is recorded.
  - Logic is added to detect when the RA is restored to its standard
    fixed offset. In this case, the tracking state is reset (ra_loc set
    to SFRAME_FRE_ELEM_LOC_NONE), deferring to the ABI's default fixed RA
    offset behavior.

gas/
	* gen-sframe.c (sframe_xlate_do_offset): Support .cfi_offset for RA
	by switching to Flex FDEs when necessary.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe-x86_64-6.d: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-6.s: New test to check
	transition of location of REG_RA from register to CFA-8 (default
	location on AMD64).  Flex FDE in effect.
	* gas/cfi-sframe/cfi-sframe.exp: Add new test.