gas: sframe: partially process DWARF unwind info in CFI_escape

gas: sframe: partially process DWARF unwind info in CFI_escape

CFI_escape is most commonly used to include DWARF expressions in the
unwind information.  One may also use CFI_escape to add OS-specific CFI
opcodes.  Up until now, SFrame generation process would skip generating
SFrame FDE at the mere sight of a CFI_escape opcode.

Fine tune the handling of CFI_escape for SFrame generation by explicitly
checking for few "harmless" (in context of SFrame generation)
CFI_escape DWARF info:
  - DW_CFA_expression affecting registers of no significance to SFrame
    stack trace info
  - DW_CFA_value_offset affecting registers of no significance to SFrame
    stack trace info

Expose the current cfi_escape_data structure in dw2gencfi.c to the
relevant header file to allow SFrame generation APIs to use it too.

Valid unwind info may be split across multiple .cfi_escape directives.
Conversely, it is also allowed to simply put multiple DWARF expressions
and/or operations in a single .cfi_escape directive.  Handling all of
these cases correctly will need parsing/processing that is not deemed
worth the effort in context of SFrame generation; We continue to skip
generating SFrame FDE for these cases and warn the user.

In future, SFrame stack trace format may support non-SP/FP as base
register (albeit in limited form).  Add an explicit check in
sframe_xlate_do_escape_expr (to test against the current CFA register)
to ensure the functionality continues to work.

Use differentiated warning text in sframe_xlate_do_val_offset to avoid
confusion to the user as the same function is used for handling
.cfi_val_offset and .cfi_escape DW_CFA_val_offset,...

Also, add a common test with DWARF reg 12 which is non SP / FP on x86_64
and aarch64 (and s390x too).

gas/
	* gas/dw2gencfi.c (struct cfi_escape_data): Move from ...
	* gas/dw2gencfi.h (struct cfi_escape_data): ... to.
	* gas/gen-sframe.c (sframe_xlate_do_val_offset): Include string
	for .cfi_escape conditionally.
	(sframe_xlate_do_escape_expr): New definition.
	(sframe_xlate_do_escape_val_offset): Likewise.
	(sframe_xlate_do_cfi_escape): Likewise.
	(sframe_do_cfi_insn): Handle CFI_escape explicitly.

gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: Add new tests.
	* gas/cfi-sframe/cfi-sframe-common-9.d: New test.
	* gas/cfi-sframe/cfi-sframe-common-9.s: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-1.d: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-1.s: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-2.d: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-2.s: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-3.d: New test.
	* gas/cfi-sframe/cfi-sframe-x86_64-empty-3.s: New test.