Threads: parsing of atomic load/store instructions

[MaxDesiatov]

See threads proposal explainer for more details.

These instructions are shimmed with existing non-atomic loads and stores for now, which seems reasonable as we can't spawn any threads yet. I've also added #include <stdatomic.h> placeholder to _CWasmKit, which we'll use for the actual implementation in a future PR.

[kateinoigakukun]

I expect we need real atomicity to pass the test suite but I think it's still a good starting point.

Would you mind adding a feature flag to gate it behind?

[MaxDesiatov]

I expect we need real atomicity to pass the test suite but I think it's still a good starting point.

Correct, that's why the test suite isn't enabled yet, and it apparently has some new WAST directives that we'll have to implement first.

Would you mind adding a feature flag to gate it behind?

You mean WasmFeatureSet? That one already has threads property.