fix the fragment_in_dst_padding_gets_overwritten test on s390x

on s390x 128bit types have a smaller alignment then on x86[^1]. This leads to smaller structs and therefore the write_unaligned will write outside of the structs boundary.

To fix the test, change the trailing u32 into a u8. This will generate 7 trailing padding bytes on s390x and 15 on x86_64. Also change the start of the garbage data so it will span over x,y and padding:

```
s390x:   XXXXXXXXXXXXXXXXY-------
x86_64:  XXXXXXXXXXXXXXXXY---------------
                      GGGGGGGG

```

[^1]: s390x ELF ABI Table 1.1, Page 12 https://github.com/IBM/s390x-abi

Author: fneddy

tests/ui/consts/const-eval/ptr_fragments.rs

@@ -73,13 +73,13 @@ fn fragment_in_dst_padding_gets_overwritten() {
     struct Pair {
         x: u128,
         // at offset 16
-        y: u64,
+        y: u8,
     }
 
     const C: MaybeUninit<Pair> = unsafe {
         let mut m = MaybeUninit::<Pair>::uninit();
         // Store pointer half-way into trailing padding.
-        m.as_mut_ptr().byte_add(20).cast::<&i32>().write_unaligned(&0);
+        m.as_mut_ptr().byte_add(14).cast::<&i32>().write_unaligned(&0);
         // Overwrite `m`.
         let val = Pair { x: 0, y: 0 };
         *m.as_mut_ptr() = val;