This CL adds Neon SIMD optimizations for AEC3 on ARM, resulting
in an 8 times complexity reduction. The optimizations are basically
identical to what was already in place for SSE2.
BUG=chromium:14993, webrtc:6018
Review-Url: https://codereview.webrtc.org/2834073005
Cr-Commit-Position: refs/heads/master@{#17993}