make: skip pdwtags when building with CLang

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>