message: implement ExitProcess for Windows
diff: @@ -78,6 +78,10 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a + 'mir>: crate::MiriEvalContextExt<'
  let code = this.read_scalar(args[0])?.to_i32()?;
  return err!(Exit(code));
  }
  + "ExitProcess" => {
  + let code = this.read_scalar(args[0])?.to_u32()?;
  + return err!(Exit(code as i32));
  + }
  _ => if dest.is_none() {
  return err!(Unimplemented(
  format!("can't call diverging foreign function: {}", link_name),
  