Add missing write barrier

We were missing the write barrier for class_value to cref. This should
fix the segv we were seeing in http://ci.rvm.jp/logfiles/brlog.trunk-gc-asserts.20230601-165052

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

vm_insnhelper.c

@@ -1512,6 +1512,7 @@ update_classvariable_cache(const rb_iseq_t *iseq, VALUE klass, ID id, const rb_c
     RUBY_ASSERT(BUILTIN_TYPE((VALUE)cref) == T_IMEMO && IMEMO_TYPE_P(cref, imemo_cref));
     RB_OBJ_WRITTEN(iseq, Qundef, ent->cref);
     RB_OBJ_WRITTEN(iseq, Qundef, ent->class_value);
+    RB_OBJ_WRITTEN(ent->class_value, Qundef, ent->cref);
 
     return cvar_value;
 }