[driver] Create a new -mfpmath= option, which is used to control whether clang

uses Neon instructions for single-precision FP.

-mfpmath=neon is analogous to passing llc -mattr=+neonfp.
-mfpmath=[vfp|vfp2|vfp3|vfp4] is analogous to passing llc -mattr=-neonfp.

rdar://11108618

llvm-svn: 154046