Disable pthread_exit for now (#366)

Disable pthread_exit for now (#366)

The current wasi-threads has no thread-exit functionality.
Thus it isn't straightforward to implement pthread_exit
without leaking thread context. This commit simply disables
pthread_exit for now.

Also, instead of abusing `wasi_proc_exit` for thread exit,
make `wasi_thread_start` return.

Note: `wasi_proc_exit` is supposed to terminate all threads
in the "process", not only the calling thread.

Note: Depending on the conclusion of the discussion about
`wasi_thread_exit`, we might revisit this change later.

References:
https://github.com/WebAssembly/wasi-threads/issues/7
https://github.com/WebAssembly/wasi-threads/pull/17