Add assembly for x86 to OpenH264 encoder

On Linux on a build workstation, these changes seem to give around 2.5x faster encoding (frame encode time in test goes from ~25 to ~9 ms)

We do expect alerts from our performance bots when this lands.

Credit: These build changes were written by brian@highfive.com aka bbaldino@gmail.com.

BUG=600399

Review-Url: https://codereview.chromium.org/2585733002
Cr-Commit-Position: refs/heads/master@{#445033}
1 file changed