Even on x86_64, emms has to be called if the x87 state has
been clobbered - the calling code (either within libvpx or
in a caller outside of libvpx) may be using the x87 instructions,
even though use of them isn't all that common on x86_64.
This fixes builds with clang for mingw/x86_64.
Change-Id: I1f6072835590b862bad156f17331ba65c813ddd9
