Print PPC ZERO as 0 (not r0) even on Darwin

It seems that the Darwin PPC assembler requires r0 to be written as 0 when it
means 0 (at least in lwarx/stwcx.). Fixes PR15605.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178142 91177308-0d34-0410-b5e6-96231b3b80d8