[PowerPC64] Use STN_UNDEF internally for edited relocs

It's not correct to use non-STT_TLS symbols with TLS relocation, not
that it matters much when editing relocs, but this edited reloc can be
output by --emit-relocs.  So don't use a symbol on the reloc.

	* elf64-ppc.c (ppc64_elf_relocate_section): Don't bother selecting
	a TLS section symbol for edited relocs.  Tighten TLS symbol/reloc
	match test.