Add support for gcc-compatible -mpopcntd -mno-popcntd PPC options

gcc provides -mpopcntd and -mno-popcntd for controlling the popcntd target
feature; support these options as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178235 91177308-0d34-0410-b5e6-96231b3b80d8