- Corrects WASI `UTIME_*` definitions now that the libc build has been
  fixed (see previous commit), and adds the corresponding definitions
  for Emscripten which were missing.
- Fixes `dirReadUnimplemented()`, which didn't compile.
- Prevents a dependency on `pthread_kill` from being pulled in in
  single-threaded Emscripten builds, where it isn't defined.

With these changes, Emscripten can now participate in juicy main.