message: do padding correctly
diff: @@ -302,12 +302,11 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let blksize_t_layout = this.libc_ty_layout("blksize_t")?;
  let uint32_t_layout = this.libc_ty_layout("uint32_t")?;
  - // We need to add 32 bits of padding after `st_rdev` if we are in a 64-bit platform. To do
  - // this, we store `st_rdev` as a `c_long` instead of a `dev_t`.
  - let st_rdev_layout = if this.tcx.sess.target.ptr_width == 64 {
  - long_layout
  + // We need to add 32 bits of padding after `st_rdev` if we are in a 64-bit platform.
  + let pad_layout = if this.tcx.sess.target.ptr_width == 64 {
  + uint32_t_layout
  } else {
  - dev_t_layout
  + this.layout_of(this.tcx.mk_unit())?
  };
  let imms = [
  @@ -317,7 +316,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  immty_from_uint_checked(0u128, ino_t_layout)?, // st_ino
  immty_from_uint_checked(0u128, uid_t_layout)?, // st_uid
  immty_from_uint_checked(0u128, gid_t_layout)?, // st_gid
  - immty_from_uint_checked(0u128, st_rdev_layout)?, // st_rdev
  + immty_from_uint_checked(0u128, dev_t_layout)?, // st_rdev
  + immty_from_uint_checked(0u128, pad_layout)?, // padding for 64-bit targets
  immty_from_uint_checked(access_sec, time_t_layout)?, // st_atime
  immty_from_uint_checked(access_nsec, long_layout)?, // st_atime_nsec
  immty_from_uint_checked(modified_sec, time_t_layout)?, // st_mtime
  