[GOLD] internal error in relocate, at powerpc.cc:10473

GOT relocations can refer directly to a function in a fixed position
executable, unlike ADDR64 which needs a global entry stub, or branch
relocs, which need PLT stubs.

	* powerpc.cc (is_got_reloc): New function.
	(Target_powerpc::Relocate::relocate): Use it here, exclude GOT
	relocs when looking for stubs.