Use movdqu then pavgb to allow unaligned memory for rgb subsampling code.  Allows this assembly to be used for unaligned pointers as well as aligned ones with no performance hit when memory is aligned on a modern cpu.
BUG=365
TESTED=libyuvTest.ARGBToI420_Unaligned (453 ms)
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/30679004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1116 16f28f9a-4ce2-e073-06de-1de4eb20be90