Add -m(no)-spe to clang

Summary:
r337347 added support for the Signal Processing Engine (SPE) to LLVM.
This follows that up with the clang side.

This adds -mspe and -mno-spe, to match GCC.

Subscribers: nemanjai, kbarton, cfe-commits

Differential Revision: https://reviews.llvm.org/D49754

llvm-svn: 371066