clang requires -ffp-contract=fast with -mfma to actually enable FMA instructions

With clang -mfma is not enough on its own to enable the use of FMA instructions. -ffp-contract=fast needs to be added as well. This MR automatically does this, when required, for parity with how FMA is enabled with gcc.

For reference: http://releases.llvm.org/8.0.0/tools/clang/docs/ClangCommandLineReference.html#cmdoption-clang-ffp-contract