Sobel and SobelXY Neon port.  Improved Bayer - did 8 at time version, and specialized G channel version.
BUG=201
TEST=libyuvTest.TestSobel and libyuvTest.TestSobelXY
Review URL: https://webrtc-codereview.appspot.com/1279006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@642 16f28f9a-4ce2-e073-06de-1de4eb20be90