cpu_x86: Copy CPU signature from ancestor
    
    When specifying a new CPU model in cpu_map.xml as an extension to an
    existing model, we forgot to copy the signature (family + model) from
    the original CPU model.
    
    We don't use this way of specifying CPU models, but it's still supported
    and it becomes useful when someone wants to quickly hack up a CPU model
    for testing or when creating additional variants of existing models to
    help with fixing some spectral issues.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>