Port gcc asm code in scale.cc to NaCl x86-64

BUG=253
TEST=manually run tests, trybots and ncval
R=fbarchard@google.com

Review URL: https://webrtc-codereview.appspot.com/4029005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@872 16f28f9a-4ce2-e073-06de-1de4eb20be90