ld: Cleanup sframe_decoder_init_func_bfdinfo use of reloc cookie

ld: Cleanup sframe_decoder_init_func_bfdinfo use of reloc cookie

The loop did set cookie->rel to the i-th relocation twice.  At the
beginning using the loop counter.  At the end by incrementing.  One
approach is sufficient.

Change cookie to pointer-to-const, replace cookie->rel by rel,
initialize before the loop and increment at the end, and merge the
two assertions (for cookie->rel) into one.

While at it change sec to pointer-to-const.

bfd/
	* elf-sframe.c (sframe_decoder_init_func_bfdinfo): Cleanup use
	of relocation cookie.

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