gas: Skip SFrame FDE if .cfi_val_offset specifies non-default offset

Unwinding of the stack pointer (SP) is performed using the assumed
default rule ".cfi_val_offset <SP-reg>, 0", so that SP unwinds as:

  SP = CFA

Warn if the CFI directive .cfi_val_offset is encountered for the
SP register with a different offset.

gas/
	* gen-sframe.c (sframe_xlate_do_val_offset): Skip SFrame FDE
	if non-default SP value offset.

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