Add support for gcc-compatible -mmfcrf -mno-mfcrf PPC options

gcc provides -mmfcrf and -mno-mfcrf for controlling what we call
the mfocrf target feature. Also, PPC is now making use of the
static function AddTargetFeature used by the Mips Driver code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178227 91177308-0d34-0410-b5e6-96231b3b80d8