Adds armv6 optimized variance calculation

Adds vp8_sub_pixel_variance16x16_armv6 function to encoder. Integrates
ARMv6 optimized bilinear interpolations from vp8/common/arm/armv6
and adds new assembly file for variance16x16 calculation.
 - vp8_filter_block2d_bil_first_pass_armv6   (integrated)
 - vp8_filter_block2d_bil_second_pass_armv6  (integrated)
 - vp8_variance16x16_armv6 (new)
 - bilinearfilter_arm.h (new)
Change-Id: I18a8331ce7d031ceedd6cd415ecacb0c8f3392db