This is behind the flag "--harmony-atomics", and it only works on
SharedArrayBuffers. This implementation only includes the runtime functions.
The TurboFan implementation will be next.

The draft spec for Atomics can be found here:
https://docs.google.com/document/d/1NDGA_gZJ7M7w1Bh8S0AoDyEqwDdRh4uSoTPSNn77PFk

BUG=
LOG=n

Review URL: https://codereview.chromium.org/1162503002

Cr-Commit-Position: refs/heads/master@{#28796}