kunit: tool: Implement listing of available architectures

To implement custom scripting around kunit.py it is useful to get a list of
available architectures. While it is possible to manually inspect
tools/testing/kunit/qemu_configs/, this is annoying to implement and
introduces a dependency on a kunit.py implementation detail.

Introduce 'kunit.py run --arch help' which lists all known architectures
in an easy to parse list. This is equivalent on how QEMU implements
listing of possible argument values.

Link: https://lore.kernel.org/r/20250415-kunit-list-v2-1-aa452cd317ae@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>