core.stdc.fenv: proper support for Microsoft C Runtime (Visual Studio >= 2014)

VCRT 2013 uses a minimally different <tt>feraiseexcept()</tt> implementation than this one from VCRT 2014 CTP 3 (by additionally setting the exception flags via <tt>fesetexceptflag</tt>), but I guess that doesn't make a huge difference; the rest is identical, so MSVC 2013 should be supported too.
More important are the correct constants and the implementation of 2 inline functions (to prevent linking errors).