Fix ub-int-array test for big-endian platforms
As of commit 7767cbb3b0, the tests/ui/consts/const-eval/ub-int-array.rs test is failing on big-endian platforms (in particular s390x), as the stderr output contains a hex dump that depends on endianness.
Since this point intentionally verifies the hex dump to check the uninitialized byte markers, I think we should not simply standardize away the hex dump as is done with some of the other tests in this directory.
However, most of the test is already endian-independent. The only exception is one line of hex dump, which can also be made endian-independent by choosing appropriate constants in the source code.
Since the 32bit and 64bit stderr outputs were already (and remain) identical, I've merged them and removed the stderr-per-bitwidth marker.
Fixes (again) https://github.com/rust-lang/rust/issues/105383.
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