Implement a low level mutex (and condvar?) that works in Wasm Workers?

[sbc100]

We need some kind of mutex to get libc++ to work correctly. See #26375.

Maybe not the full power of pthread_mutex/pthread_cond but something a that sits on top of the existing atomics+futex APIs that can be used to implement __libcpp_mutex_t and __libcpp_condvar_t.

[sbc100]

I didn't realize but wasm workers actually already provides API such as emscripten_lock_* and emscripten_semaphore_*