[GOLD] PowerPC64 TOC indirect to TOC relative code editing

[GOLD] PowerPC64 TOC indirect to TOC relative code editing

Doesn't yet trim off the unused TOC entries.

	* powerpc.cc (class Powerpc_copy_relocs): New.
	(Powerpc_copy_relocs::emit): New function.
	(Powerpc_relobj::relatoc_, toc_, no_toc_opt_): New variables.
	(Powerpc_relobj::toc_shndx, set_no_toc_opt, no_toc_opt): New inlines.
	(Powerpc_relobj::do_relocate_sections): New function.
	(Powerpc_relobj::make_toc_relative): Likewise.
	(Powerpc_relobj::do_find_special_sections): Stash away .rela.toc
	and .toc too.
	(ok_lo_toc_insn): Move earlier, and handle more insns.
	(Target_powerpc::Scan::local): If optimizing toc accesses, set
	no_toc_opt for entries we can't edit.  Check insn validity.
	Emit "toc optimization is not supported" warning, downgraded
	from error.
	(Target_powerpc::Scan::global): Likewise.
	(Target_powerpc::Relocate::relocate): Edit TOC indirect code
	to TOC relative.  Don't emit "toc optimization is not supported"
	error here.