Fix performance regression due to ld -r memmove

Fix performance regression due to ld -r memmove

The idea here is that instead of using memmove to shuffle the relocs
array every time one is deleted, to add a "wrel" pointer and copy from
rel[0] to wrel[0] as we go.

	* elf64-ppc.c (ppc64_elf_relocate_section): Use read and write
	pointers to reloc array, rather than memmove when deleting a
	reloc.  Don't use RELOC_AGAINST_DISCARDED_SECTION.  Adjust
	reloc counts at end of loop.
	* elf32-ppc.c (ppc_elf_relocate_section): Likewise.