Shenandoah: Allow VM global oop storage to be processed concurrently

JDK-8227653 added new VM global oop storage, and piggyback it to SystemDictionary for oop processing. This is not desirable for Shenandoah, since SystemDictionary is defined as a serial root, that can only be processed at paused.

VM global oop storage should be implemented in the same way as JNIHandleRoots, which allows it to be processed concurrently if possible.