[PowerPC] Implement writeNopData

This implements a proper PPCAsmBackend::writeNopData routine
that actually writes PowerPC nop instructions.

This fixes the last remaining difference in object file output
(text section) between the integrated assembler and GNU as
that I've seen anywhere.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185662 91177308-0d34-0410-b5e6-96231b3b80d8