[ARM]: Extend -mfpu options for half-precision and vfpv3xd

Some of the the permissible ARM -mfpu options, which are supported in GCC,
are currently not present in llvm/clang.This patch adds the options:
'neon-fp16', 'vfpv3-fp16', 'vfpv3-d16-fp16', 'vfpv3xd' and 'vfpv3xd-fp16.
These are related to half-precision floating-point and single precision.

Reviewers: rengolin, ranjeet.singh

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10645

llvm-svn: 240930