Shenandoah: More consistent naming of LRB runtime entry points

Current naming of LRB runtime entry points is a bit weird. We have:

- load_reference_barrier_JRT (where _JRT doesn't serve any purpose)
- oop_load_from_native_barrier (which is really like LRB except loading from native memory)

I propose to rename to:
- load_reference_barrier
- load_reference_barrier_native