PowerPC64, don't relocate nops

This fixes a "bug" in that nops emitted as part of code optimization
were being relocated.  As it happens the relocation value was always
zero so the nop wasn't changed.  Whew!  I've also moved the use of
"howto" later since I was caught out in some recent code changes with
the howto not matching r_type.

	* elf64-ppc.c (ppc64_elf_relocate_section): Don't relocate nops
	emitted for toc sequence optimization.  Set and use "howto" later.