Add optimized Neon functions of:
vp9_variance32x64
vp9_variance64x32
vp9_variance64x64

On Nexus 7 speed -5 and -6 saw about a 4% increase in perf.
Speeds -7 and -8 saw about a 6% increase in perf.
Tested on Nexus 7, built with ndk r10d, gcc 4.9.

Change-Id: I5a81f13c9897eb927fa39662530f5524a0f768fa