Only target 32bit builds. Visual Studio does not define _mm_empty for
64bit configurations.

Rename emms.asm and remove from 32 bit builds to avoid empty file
warnings.

Don't check register state on 64bit builds.

BUG=webm:1500

This reverts commit 60beb781.

Change-Id: I5ac4cf6c67249ff24f7da19792144de20527bfce