Lower __builtin_fabs* to @llvm.fabs.*

Lower __builtin_fabs* to @llvm.fabs.*

mingw64's headers implement fabs by calling __builtin_fabs, so using the
library call results in an infinite loop. If the backend legalizes
@llvm.fabs as a call to fabs later, things should work out, as the crt
provides a definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221206 91177308-0d34-0410-b5e6-96231b3b80d8