CGBuiltin: Use @llvm.fabs rather than fabs libcall when emitting builtins

Using the intrinsic allows the SelectionDAGBuilder to turn this call
into the FABS Node and also the intrinsic is something the vectorizer knows
how to vectorize.

This patch also sets the readnone attribute on this call, which should
enable additional optmizations.

llvm-svn: 217042