_bfd_mips_elf_lo16_reloc vallo comment

This explains exactly why the high reloc adjustment is as it is,
replacing the rather nebulous existing comment.  I've also changed the
expression from (lo+0x8000)&0xffff to (lo&0xffff)^0x8000 which better
matches part of the standard 16-bit sign extension (resulting in
exactly the same value), and hoisted the calculation out of the loop.

	* elfxx-mips.c (_bfd_mips_elf_lo16_reloc): Expand vallo
	comment.  Hoist calculation out of loop.