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 agree with @kinke here, there should be a proper strategy using c_long_double, but not to be decided in this PR. Right now, it would not be used, because you have to add it explicitely at the call site. If there is implicite conversion, it would probably introduce ambiguities.
The problem here is not mangling, but overload resolution for real.