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.
Okay so how about this as work-around:
```diff
-real acoshl(real x);
+extern(D) real acoshl(real x) { return acosh(cast(double) x); }
```
etc., i.e., adding a dedicated *l wrapper working with proper D reals and using D name mangling so as not to collide with the C function in MSVCRT.
All MSVCRT *l functions working with their 64-bit C ```long double``` are completely useless for us as long as D ```real``` remains defined as 80-bit on Windows x64 and including them in druntime would only make sense if we relaxed it to 64-bit.
The proposed D wrappers would at least feature the expected signature for the time being.