return from `_start`

[yamt]

a. some people think a return from _start should be an equivalent of thread_exit. (ie. same as a return from wasi_thread_start)

b. other people think it should be an equivalent of proc_exit(0). (ie. it should terminate other threads.)

c. maybe "leave it undefined for threaded apps for now" is also a valid choice if we make wasi-libc _start call proc_exit explicitly.

i guess it's up to wasi-threads to decide either ways because, without threads, thread_exit and proc_exit are basically same.

cf. WebAssembly/wasi-libc#367