Salsa CI: Add arm64 and armhf builds

As an arm64 runner is now available in the Salsa group 'mariadb-team' start building on 'arm64' and 'armhf' architectures. Don't build on 'armel' as it fails to compile on the Salsa-CI runner (potentially due to cross-build issues).

Use custom "if" rules to ensure forks of this project are not affected and will not get stuck jobs, as forks are unlikely to have custom 'arm64' tagged runners.