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 60beb781c140b61c1957abd2a6717d2e9a831933.
Change-Id: I5ac4cf6c67249ff24f7da19792144de20527bfce