PowerPC64 ha/lo insn checks

PowerPC64 ha/lo insn checks

These are done in ppc64_elf_edit_toc, which now also garbage collects
unused GOT entries.  The checks for legitimate instructions weren't
being done for the GOT relocs, unless the file also happened to have a
toc section.

	* elf64-ppc.c (struct ppc64_elf_obj_tdata): Rename has_gotrel
	to has_optrel.
	(struct _ppc64_elf_section_data): Likewise.
	(ppc64_elf_check_relocs): Set has_optrel for more relocs.
	(ppc64_elf_edit_toc): Do ha/lo insn checks in GOT loop rather
	than TOC loop.  Check PLT16 insns too.