ARM32/64 NEON versions of ScaleARGBRowDown2Linear_NEON are implemented.

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

Change-Id: Ife602c81b51aa36e0d56b9d628f278a24eed96f6

Review URL: https://webrtc-codereview.appspot.com/44689004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1336 16f28f9a-4ce2-e073-06de-1de4eb20be90