This adds decoding and compilation of the "atomic.fence" operator, which
is intended to preserve the synchronization guarantees of higher-level
languages.

Unlike other atomic operators, it does not target a particular linear
memory. It may occur in modules which declare no memory, or a non-shared
memory, without causing a validation error.

See proposal: https://github.com/WebAssembly/threads/pull/141
See discussion: https://github.com/WebAssembly/threads/issues/140

R=clemensh@chromium.org
TEST=cctest/test-run-wasm-atomics/RunWasmXXX_AtomicFence
BUG=v8:9452

Change-Id: Ibf7e46227f7edfe5c81c097cfc15924c59614067
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1701856
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62821}