Make static constants const for Visual C.  Allows optimizer to collapse similar constants (SSE2 is subset of AVX2).  GCC has compiler bug with const attributes, but Visual C does not.
BUG=254
TEST=windows build still passes and is smaller
R=cdelaunay@google.com

Review URL: https://webrtc-codereview.appspot.com/2200007

git-svn-id: http://libyuv.googlecode.com/svn/trunk@788 16f28f9a-4ce2-e073-06de-1de4eb20be90