message: implement some libc hooks needed by libtest
diff: @@ -426,6 +426,7 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
  let paths = &[
  (&["libc", "_SC_PAGESIZE"], Scalar::from_int(4096, dest.layout.size)),
  (&["libc", "_SC_GETPW_R_SIZE_MAX"], Scalar::from_int(-1, dest.layout.size)),
  + (&["libc", "_SC_NPROCESSORS_ONLN"], Scalar::from_int(1, dest.layout.size)),
  ];
  let mut result = None;
  for &(path, path_value) in paths {
  @@ -452,6 +453,10 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
  }
  }
  + "isatty" => {
  + self.write_null(dest)?;
  + }
  +
  // Hook pthread calls that go to the thread-local storage memory subsystem
  "pthread_key_create" => {
  let key_ptr = this.read_scalar(args[0])?.to_ptr()?;
  