make: skip pdwtags when building with CLang
    
    When building with CLang the structs that are emitted by pdwtags appear
    in a completely different order than with GCC, which causes the
    comparison against expected data to fail.
    
    Ideally the test would not be sensitive to the ordering, because even
    future GCC could cause changes, but that's not easy to fix. So for now
    just skip the test when using clang.
    
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>