Replace always_update_barrier_set with proper API in BarrierSet

There's a global variable always_update_barrier_set that directs whether or not oop stores and its GC barriers require a release membar. It is only used for CMS. It should be refactored into a proper API in BarrierSet, so that this GC specific hook can be avoided in shared code, and instead be implemented by GCs that need it.