ARM32/64 NEON versions of ScaleRowDown2Linear_NEON are implemented.

BUG=319
TESTED=libyuvTest.ScaleDownBy2_Linear on ARM32/64 with Android
R=fbarchard@google.com

Change-Id: I2c7f43a0d56ed4dfded5bdbbb61765d87d65a2ba

Review URL: https://webrtc-codereview.appspot.com/43519005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1307 16f28f9a-4ce2-e073-06de-1de4eb20be90