Move Universe roots into OopStorage

These can go in vm_global() oop storage since they're global and don't change once created.
Things that need special handling are:
   the mirrors are saved/restored with CDS
   _reference_pending_list is used atomically
   _the_null_sentinel is referenced by generated code

Universe::oops_do() also calls
   ThreadsSMRSupport::exiting_threads_oops_do(f);

So removing Universe as a case in all the GCs is blocked on changing Thread::_threadObj to an OopHandle.