Add support for gcc-compatible -mfprnd -mno-fprnd PPC options

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178414 91177308-0d34-0410-b5e6-96231b3b80d8