8296875: Generational ZGC: Refactor loom code

Co-authored-by: Stefan Karlsson <stefank@openjdk.org>
Co-authored-by: Axel Boldt-Christmas <aboldtch@openjdk.org>
Reviewed-by: stefank, rrich, pchilanomate

src/hotspot/share/gc/shared/modRefBarrierSet.hpp

@@ -40,6 +40,7 @@ protected:
                  barrier_set_c1,
                  barrier_set_c2,
                  NULL /* barrier_set_nmethod */,
+                 NULL /* barrier_set_stack_chunk */,
                  fake_rtti.add_tag(BarrierSet::ModRef)) { }
   ~ModRefBarrierSet() { }