fmod for 64bit is slow

[simonbyrne]

The fmod function on x86_64 is pretty slow (e.g. roughly 5x slower than remainder, or fmod in the os x libm). From what I understand of the Makefile, it is using the src/e_fmod.c, which is implemented in software. The other remainder functions (remainder, remquo) use the assembly implementations in amd64 (which use x87 instructions).

I should mention that this isn't a problem for Julia at the moment, as we access fmod through the LLVM intrinsic, which in turn calls the system libm. But if this were to change, we would see a significant performance hit.