Fix a data race for test main_proc_exit_wait.c from #1963.
And fix atomic_wait logic that was wrong before:
- a thread 1 started executing wasm instruction wasm_atomic_wait
but hasn't reached waiting on condition variable
- a main thread calls proc_exit and notifies all the threads that reached
waiting on condition variable
Which leads to thread 1 hang on waiting on condition variable after that
Now it's atomically checked whether proc_exit was already called.
