core.stdc.math: proper support for Microsoft C Runtime (Visual Studio >= 2013)

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.
The new commit implements ```signbit(real)``` and uses an ```assert(false)``` in ```fpclassify(real)``` for now.