This adds support for the I32AtomicCompareExchange operations in the
interpreter. Also, the interpreter will now fail if it encounters
an unknown opcode from the atomic prefix.

Bug: chromium:826069
Change-Id: Iec1742271f4fdd83fcaa09ca72c24d1cf8c58835
Reviewed-on: https://chromium-review.googlesource.com/1029867Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52807}