core.stdc.stdio: prepare for MSVCRT 2014 support

Once MSVC 2014 is required, things will get easier, for example here because they now provide a standard-conformant <tt>(v)snprintf()</tt> implementation, see [this MSDN blog](http://blogs.msdn.com/b/vcblog/archive/2014/06/18/crt-features-fixes-and-breaking-changes-in-visual-studio-14-ctp1.aspx).
Additionally, <tt>_fputc_nolock()</tt> and <tt>_fgetc_nolock()</tt> are no longer defined as macros, but available in the library, just like in any other supported C runtime.