When upgrading our cross gcc packages, build some packages with them to
ensure they work as expected.
Put the gitlab-ci.yml in cross/ instead of cross/gcc-x86_64, because:
* it tests gcc-aarch64 too
* so it does not get removed when regenerating gcc-x86_64 with
pmbootstrap
Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6776