Reason of this bug is that ARM64 can't extend the sign bit of a 32-bit integer
to 64-bit integer automatically.
ScaleARGBRowDownEven_NEON is also enabled for ARM64.

BUG=319
TESTED=libyuv_unittest
R=fbarchard@google.com

Change-Id: Ib8d30a05156239247296aa8bb4faa94b4f69a9c3

Review URL: https://webrtc-codereview.appspot.com/32949004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1273 16f28f9a-4ce2-e073-06de-1de4eb20be90