gas: Use SFrame header and FDE field sizes when generating .sframe

The use of SFRAME_RELOC_SIZE in generation of SFrame stack trace
information from CFI directives erroneously suggested that this could
be used to configure a different relocation size.  But in practice it
is tied to the SFrame field sizes it is used for and therefore cannot
be changed.

Replace the uses of SFRAME_RELOC_SIZE by the size of the respective
SFrame header and FDE fields when emitting SFrame information.  While
at it enhance some comments.

gas/
	* gen-sframe.c (SFRAME_RELOC_SIZE): Delete.
	(sizeof_member): Define.
	(output_sframe_funcdesc): Use size of SFrame FDE fields instead
	of SFRAME_RELOC_SIZE.
	(output_sframe_internal): Use size of SFrame header fields
	instead of SFRAME_RELOC_SIZE.

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