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 (24 instead of 32 bytes) 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
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