don't inhibit random field reordering on repr(packed(1))

Author: RalfJung

tests/fail/reading_half_a_pointer.rs

@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 
 // We use packed structs to get around alignment restrictions
-#[repr(packed)]
+#[repr(C, packed)]
 struct Data {
     pad: u8,
     ptr: &'static i32,

tests/fail/unaligned_pointers/field_requires_parent_struct_alignment2.rs

@@ -7,7 +7,7 @@ pub struct Aligned {
     _pad: [u8; 11],
     packed: Packed,
 }
-#[repr(packed)]
+#[repr(C, packed)]
 #[derive(Default, Copy, Clone)]
 pub struct Packed {
     _pad: [u8; 5],