Windows port: fix crash using fmod() on Windows 32 bit.

 This generates an instruction which doesn't seem to be supported, probably need compiler-rt
for this but I couldn't get it working, so now it's compiled as a function.

Relevant LLVM reports:
http://llvm.org/bugs/show_bug.cgi?id=5053
http://llvm.org/bugs/show_bug.cgi?id=5052
http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053917.html

I'm not happy about this fix, but it works and I haven't gotten reports about other crashes.