libvpx

VP8 for ARMv8 by using NEON intrinsics 17

Add vp8_subpixelvariance_neon.c
- vp8_sub_pixel_variance16x16_neon_func
- vp8_variance_halfpixvar16x16_h_neon
- vp8_variance_halfpixvar16x16_v_neon
- vp8_variance_halfpixvar16x16_hv_neon
- vp8_sub_pixel_variance8x8_neon

Change-Id: I3e5d85b2eafc26be0eef6a777789b80e4579257b
Signed-off-by: James Yu <james.yu@linaro.org>