First step towards more human-friendly PPC assembler output:

- add -ppc-reg-with-percent-prefix option to use %r3 etc as register
  names
- split off logic for Darwinish verbose conditional codes into a helper
  function
- be explicit about Darwin vs AIX vs GNUish assembler flavors

Based on the patch from Alexandre Yukio Yamashita

Differential Revision: https://reviews.llvm.org/D39016

llvm-svn: 319381