gas: sframe: pass SFrame FDE as argument to output_sframe_row_entry

Minor refactoring.  Will be needed later for supporting flexible topmost
frames in SFrame V3.

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

gas/
	* gen-sframe.c (output_sframe_row_entry): Use new argument.
	(output_sframe_internal): Likewise.