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Surprisingly this hadn't been done. So we are missing SSE, SSE2
optimizations which should be available on all platforms, and a
couple SSE4.1 optimizations which are runtime selected.

This condenses all include_dirs, cflags, and defines into a common
opus_private_config for sharing with intrinsic targets.

It looks like AVX support has been started in libopus, so the next
update that pulls in some of that code will need to add a opus_avx
target tagged with a -mavx cflag.

This doesn't seem to add any decoding speedup per media_perftests,
but much of this code is for encoding, so WebRTC and MediaRecorder
should benefit.

BUG=none
TEST=unittests all pass and bots are happy

Change-Id: I0a12460af899c4d94c16e4024c043ff25e896463
Reviewed-on: https://chromium-review.googlesource.com/c/1318807Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606131}