Shenandoah: Should evacuate/update codecache concurrently when class unloading is off

During isolating GC work (JDK-8255765), I found that Shenandoah still evacuate/update codecache roots at final mark safepoint when class unloading is off.

Actually, it can be done concurrently by utilizing nmethod_entry_barrier.

Shenandoah should always enable nmethod_entry_barrier for evacuation, regardless class unloading status, and perform code cache evacuation/updating during strong root processing.