Add 64x64 sub_pel_variance Neon function

On Nexus 7 speed -5, -6, -7, and -8 saw about a 15% increase
in perf for 480p. Speeds -5, -6, -7, and -8 saw about a 10%
increase in perf for 720p.

Tested on Nexus 7, built with ndk r10d, gcc 4.9.

Change-Id: I2fa5315845e3021c9a6e2ea47e52e68b398d8334