Support for acquire/release semantics in atomics

[stevenfontanella]

Following the shared everything threads proposal, Wasm will add support for relaxed atomic memory operations with a new memory ordering for acquire/release in addition to the existing sequentially consistent mode.

We should add support for reading/writing the new memory orderings. For now, this won't affect our optimizations; we will conservatively treat acquire/release memory operations the same as sequentially consistent.

• Parsing/printing/execution for atomic instructions with the new immediate
• C + JS API Changes
• Feature flag for relaxed atomics
• Fuzzing support
• Spec tests in Binaryen / the proposal