gas: Don't skip SFrame FDE if .cfi_register specifies SP register

gas: Don't skip SFrame FDE if .cfi_register specifies SP register

Neither ".cfi_offset SP, <offset>", ".cfi_register SP, <regno>", nor
".cfi_val_offset SP, <offset>" alter the tracking information to recover
the stack pointer (SP). Doing so would need an explicit .cfi_def_cfa,
which SFrame tracks.

The stack pointer (SP) register contents on entry can be reconstructed
from the SFrame CFA tracking information using information from the
current and initial SFrame FREs of the SFrame FDE:

1. Compute CFA from the current CFA base register (SP or FP) and CFA
   offset from the SFrame CFA tracking information from the SFrame FRE
   for the current instruction address:

     CFA = <current_base_reg> + <current_cfa_offset>

2. Compute SP from the current CFA and the CFA offset from the SFrame
   CFA tracking information from the initial SFrame FRE of the FDE:

     SP = CFA - <initial_cfa_offset>

While at it add comments to the processing of .cfi_offset and
.cfi_val_offset that the SP can be reconstructed from the CFA tracking
information.

gas/
	* gen-sframe.c (sframe_xlate_do_register): Do not skip SFrame
	FDE if .cfi_register specifies SP register.
	(sframe_xlate_do_offset,sframe_xlate_do_val_offset): Add comment
	that this is likewise.

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