Disable floating point exception checking for clang.

Clang and LLVM currently completely ignore any FPE settings when determining
optimization passes. This can lead to re-introduction of FPEs that have been
guarded against in if-statements when the optimizer re-orders statements.

See https://llvm.org/bugs/show_bug.cgi?id=24807
and https://llvm.org/bugs/show_bug.cgi?id=18673.