Support grabbing the MXCSR mask on x86_64.

Instead of using some dynamic code to grab the FXSave information, use an intrinsic if on at least MSVC 2012.
With GCC just use a bit of ASM, and if on MSVC 2010 or older, use the old crappy method.

This method can be removed once MSVC 2010 support is dropped and mandate at least MSVC 2012 minimum.