work arounds for ios 64 bit compiler where int passed into assembly needs to be explicitely cast to 'w' register.
BUG=437
TESTED=local ios build
R=bcornell@google.com

Review URL: https://webrtc-codereview.appspot.com/49289004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1402 16f28f9a-4ce2-e073-06de-1de4eb20be90