Adds following ARMv6 optimized functions to the encoder:
 - vp8_variance_halfpixvar16x16_h_armv6
 - vp8_variance_halfpixvar16x16_v_armv6
 - vp8_variance_halfpixvar16x16_hv_armv6

Change-Id: I1e9c2af7acd2a51b72b3845beecd990db4bebd29