threads: implement support for unnamed semaphores (#316)

threads: implement support for unnamed semaphores (#316)

[POSIX semaphores] come in two forms: named and unnamed. Roughly, named
semaphores use files to implement locking across processes; unnamed
semaphores use a shared memory region to implement locking across
threads in the same process. Since WASI currently has no process concept
(and it is relatively unclear how to map the WASI files as shared
memory), only the unnamed semaphores are supported by this changed. This
means that `sem_open`, `sem_close`, and `sem_unlink` will not available
to programs compiled with a threads-enabled `wasi-libc`.

[POSIX semaphores]: https://man7.org/linux/man-pages/man7/sem_overview.7.html