message: Fix dirent layout for macOS
diff: @@ -950,7 +950,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let c_ushort_layout = this.libc_ty_layout("c_ushort")?;
  let c_uchar_layout = this.libc_ty_layout("c_uchar")?;
  - let name_offset = dirent_layout.details.fields.offset(4);
  + let name_offset = dirent_layout.details.fields.offset(5);
  let name_ptr = entry_ptr.offset(name_offset, this)?;
  #[cfg(unix)]
  @@ -969,8 +969,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let imms = [
  immty_from_uint_checked(ino, ino_t_layout)?, // d_ino
  - immty_from_uint_checked(0u128, off_t_layout)?, // d_off
  + immty_from_uint_checked(0u128, off_t_layout)?, // d_seekoff
  immty_from_uint_checked(0u128, c_ushort_layout)?, // d_reclen
  + immty_from_uint_checked(file_name.len() as u128, c_ushort_layout)?, // d_namlen
  immty_from_uint_checked(file_type, c_uchar_layout)?, // d_type
  ];
  this.write_packed_immediates(entry_place, &imms)?;
  