libstdc++: Use UTF-32BE as wide encoding for big-endian machines [PR119725]

libstdc++: Use UTF-32BE as wide encoding for big-endian machines [PR119725]

This changes the `dg-options` line so UTF-32 with byte order native to the
machine is used as wide encoding.

We still do not handle mismatch in the byte order of the Unicode encodings
(UTF32-BE on little-endian machines). This would require larger changes,
as for example `unicode-data.h` tables are encoded with native byte order.

	PR libstdc++/119725

libstdc++-v3/ChangeLog:

	* testsuite/std/format/debug.cc: Updated dg-options.
	* testsuite/std/format/debug_nonunicode.cc: Updated dg-options.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>