Rollup merge of #106047 - uweigand:s390x-test-bigendian-ui, r=oli-obk

Fix ui constant tests for big-endian platforms

A number of tests under ui/const-ptr and ui/consts are currently failing on big-endian platforms as the binary encoding of some constants is hard-coded in the stderr test files.

Fix this by a combination of two types of changes:

- Where possible (i.e. where the particular value of a constant does not affect the purpose of the test), choose constant values that have the same encoding on big- and little-endian platforms.

- Where this is not possible, provide a normalize-stderr-test rule that transforms the printed big-endian encoding of such constants into the corresponding little-endian form.

Fixes part of https://github.com/rust-lang/rust/issues/105383.