Shenandoah: GC workers should always get the largest GCLABs

We have the adaptive GCLAB sizing that starts with min TLAB size and doubles up on every refresh. That is suitable for Java threads that rarely evacuate through the LRB slowpath. GC workers, on the other hand, do the bulk of the GC evacuation work, and should just use the largest GCLABs available for them.