message: fix warnings for non-Unix builds
diff: @@ -549,12 +549,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  target_op: OpTy<'tcx, Tag>,
  linkpath_op: OpTy<'tcx, Tag>
  ) -> InterpResult<'tcx, i32> {
  - #[cfg(target_family = "unix")]
  + #[cfg(unix)]
  fn create_link(src: &Path, dst: &Path) -> std::io::Result<()> {
  std::os::unix::fs::symlink(src, dst)
  }
  - #[cfg(target_family = "windows")]
  + #[cfg(windows)]
  fn create_link(src: &Path, dst: &Path) -> std::io::Result<()> {
  use std::os::windows::fs;
  if src.is_dir() {
  @@ -816,7 +816,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  this.check_no_isolation("mkdir")?;
  - let _mode = if this.tcx.sess.target.target.target_os == "macos" {
  + #[cfg_attr(not(unix), allow(unused_variables))]
  + let mode = if this.tcx.sess.target.target.target_os == "macos" {
  u32::from(this.read_scalar(mode_op)?.not_undef()?.to_u16()?)
  } else {
  this.read_scalar(mode_op)?.to_u32()?
  @@ -824,14 +825,15 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let path = this.read_path_from_c_str(this.read_scalar(path_op)?.not_undef()?)?;
  + #[cfg_attr(not(unix), allow(unused_mut))]
  let mut builder = DirBuilder::new();
  // If the host supports it, forward on the mode of the directory
  // (i.e. permission bits and the sticky bit)
  - #[cfg(target_family = "unix")]
  + #[cfg(unix)]
  {
  use std::os::unix::fs::DirBuilderExt;
  - builder.mode(_mode.into());
  + builder.mode(mode.into());
  }
  let result = builder.create(path).map(|_| 0i32);
  