Change cpu id constants to hex to indicate they are flags.  Reserve values for future flags.  make unittest print in hex.  Add environment variables to disable sse42 and avx.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/608006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@272 16f28f9a-4ce2-e073-06de-1de4eb20be90