Driver: Link crtfastmath.o if it's available and -ffast-math is specified.

crtfastmath.o contains routines to set the floating point flags to a faster,
unsafe mode. Linking it in speeds up code dealing with denormals significantly
(PR14024).

For now this is only enabled on linux where I can test it and crtfastmath.o is
widely available. We may want to provide a similar file with compiler-rt
eventually and/or enable it on other platforms too.

llvm-svn: 165240