kernel/synflood: disable on s390x
    
    There are no s390x machines with enough cores to run this test in RH's
    Beaker and likely won't be available in any CI system in the near
    future. Thus, exclude the test from this arch so that testing doesn't
    fail due to lack of matching hardware.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>