support running per architecture
    
    Instead of running on all supported architectures in a single run,
    support running on a specific chosen architecture that is passed in
    as taskotron_arch. The former approach is still available, if you pass
    'noarch' as architecture.
    
    Instead of examining noarch and src packages every time, examine them
    only with x86_64 rpms (which should be executed always, since it's a
    primary arch).