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 think only supporting the latest version of the MS C runtime is not going to work, especially as they are considering higher update frequency for VS.

Agreed, the link errors are not great, maybe we can mitigate the frustration by
* adding comments about the first supported C runtime version or 
* adding version conditionals that you could pass on the command line of your project to restrict declared functions, e.g. -version=MSVCRT10 would exclude acosh.