As per the preparation patch added in Chromium sources at,
2150943003: Add MIPS SIMD Arch (MSA) build flags for GYP/GN builds

This patch adds first MSA optimized function in libYUV project.

BUG=libyuv:634
R=fbarchard@google.com

Review URL: https://codereview.chromium.org/2285683002 .