Raising "wasi proc exit" exception, spreading it to other threads and then
clearing it in all threads may result in unexpected behavior: the sub thread
may end first, handle the "wasi proc exit" exception and clear exceptions
of other threads, including the main thread. And when main thread's
exception is cleared, it may continue to run and throw "unreachable"
exception. This also leads to some assertion failed.
Ignore exception spreading for "wasi proc exit" and don't clear exception
of other threads to resolve the issue.
And add suspend flag check after atomic wait since the atomic wait may
be notified by other thread when exception occurs.
The example wasn't fully implemented the intention - it didn't work as
expected when the trap/proc_exit was executed on the main thread,
because main thread never waited for all the threads to start.
