core.stdc.math: proper support for Microsoft C Runtime (Visual Studio >= 2014)

These commits prepare <tt>core.stdc.math</tt> for full support of Microsoft's upcoming runtime with improved C99 support. The annoying differences between 32 and 64 bit platforms are taken care of.
It'll also be relevant for LDC.
Based on the C headers from VS 2014 CTP 3.
Hi Rainer, I'd prefer getting rid of most existing MS workarounds (I'm not talking about Windows, but the - at least until now - crappy C99 support in their runtime) bloating druntime and force D developers to code in a portable way - in this case, by using the standard C99 functions if not something from Phobos.