disable 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.

For now disable the tests on s390x. [^2]

[^1]: s390x ELF ABI Table 1.1, Page 12 https://github.com/IBM/s390x-abi
[^2]: #149056 (comment)

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: fneddy

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

@@ -68,6 +68,7 @@ const _PARTIAL_OVERWRITE: () = {
 };
 
 #[allow(dead_code)]
+#[cfg(not(target_arch = "s390x"))] // u128 is less aligned on s390x, removing the padding
 fn fragment_in_dst_padding_gets_overwritten() {
     #[repr(C)]
     struct Pair {