scale ported to OSX for 32/64 bit.  Required breaking up some functions into 2 or 3 asm inlines to set constants.  Reworked storing of 6 bytes on all platforms for 3/4 scale by storing first 4 bytes and then storing last 4 bytes, overlapping 2, in order to stay within SSE2 registers.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/582004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@266 16f28f9a-4ce2-e073-06de-1de4eb20be90