use preferred arch if there is more options
    
    https://pagure.io/koji/pull-request/323 introduced option to force an
    arch for noarch packages. Nevertheless, it is only task architecture and
    not buildroot one. So, if builder can provide more possible archs (32/64
    bit), it will pick one of these randomly and can go against preferred arch.
    
    Patch introduces "preferred_arch" option for find_arch and use the
    task's one when it works with noarch rpm.
    
    Fixes: https://pagure.io/koji/issue/789