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.
I don't like it. Yes, the autotester may yield green due to missing unit tests, but I still very much prefer a clean requirement of MSVCRT >= 2013 (by producing linking errors when building druntime) rather than getting linking errors as soon as I actually call these functions in my code because that can get very frustrating, one simply expects a complete druntime/Phobos after building everything successfully.