threads: implement support for pthread mutexes (#315)

threads: implement support for pthread mutexes (#315)

This change adds pthread's mutex support to the `THREAD_MODEL=posix`
build of wasi-libc. Some less-common features are unsupported and
documented here:
- mutex robust lists are disabled and their use will return a runtime
  error; currently WASI does not support the concept of multiple
  processes so maintaining robust mutexes across processes does not yet
  make sense in wasi-libc
- timed locks with priority inheritance (PI) are disabled and will act
  as any other mutex; this feature is related to task priorities which
  is not yet relevant in the WASI ecosystem (see [priority-inheritance
  futexes](https://man7.org/linux/man-pages/man2/futex.2.html))
- thread cancellation is ignored; this feature is difficult to support
  and @sunfishcode would likely want to discuss this before adding it at
  some later time.