yumrepoinfo: specify all primary and alternate arches
    
    There wasn't much point for 'arches' key before. Now it's more clearly
    distinguished between primary and alternate arches, and all arches
    currently available in Fedora are added (so that you can now request URL
    for s390x and you'll get the right value).
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1172