Add NEON intrinsics version for WebRtcSpl_DownsampleFastNeon.

WebRtcSpl_DownsampleFastNeon is added. SplTest in common_audio_unittests
is passed on ARM32/ARM64 platform.

BUG=4002
R=andrew@webrtc.org, jridges@masque.com

Change-Id: Ic43f5452eb7e555b998b1d1e79a9e1530be5c948

Review URL: https://webrtc-codereview.appspot.com/24359004

Patch from Yang Zhang <yang.zhang@arm.com>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@7856 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed