Avoid repeated SystemDictionaryShared::should_be_excluded calls

SystemDictionaryShared::should_be_excluded is called during CDS dump time to see if a class should be excluded from the archive. This function calls itself to check super types. As a result, some classes many be checked more than once.

The repeated calls slows down dumping. Also, If a class is excluded, we will see multiple warning messages like this, which is too verbose and confusing.

Skipping org/eclipse/osgi/util/NLS: Signed JAR
Skipping org/eclipse/osgi/util/NLS: Signed JAR
Skipping org/eclipse/osgi/util/NLS: Signed JAR
Skipping org/eclipse/osgi/util/NLS: Signed JAR