Use of atomics instructions requires the support of threads proposal and
it's not widely supported yet. So we should enable actual atomic
operations only when targeting wasm32-uknown-wasip1-threads.
This change fixes the following build error happening on Wasm:
```
error: referencing instance method '_wait(expected:)' on 'Atomic' requires the types '_MutexHandle.State' and 'UInt32' be equivalent
```
