Previously, ZResurrection::is_blocked() and ZResurrection::unblock() used loadload and storestore barriers to synchronize between the GC and mutator load barriers.

JDK-8230661 changed so that we always perform a handshake before the ZResurrection::unblock() call.

After that change we can rely on the handshake to perform the necessary synchronization, and we can change the implementation to use Atomic::load and Atomic::store.