PowerPC64: Avoid useless work on R_PPC64_TPREL34

_bfd_elf_ppc_at_tprel_transform doesn't handle prefix instructions,
and I'm not inclined to implement code editing for them.

	* elf64-ppc.c (ppc64_elf_relocate_section): Don't attempt tprel
	transform for R_PPC64_TPREL34.