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

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

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

1. Compute CFA from the current CFA base register (SP or FP) and CFA
   offset from the SFrame CFA base register 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 a comment to the processing of .cfi_val_offset that the
SP can be reconstructed from the CFA base register tracking information.

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

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