Introduce event_hook_mutex for RUBY_INTERNAL_EVENT_FREEOBJ

We can stop using rb_gc_worker_thread_set_vm_context because rb_gc_event_hook
now uses rb_gc_get_ec since ruby/ruby#16880

Author: peterzhu2118

gc/mmtk/mmtk.c

@@ -39,6 +39,8 @@ struct objspace {
     pthread_cond_t cond_world_started;
     size_t start_the_world_count;
 
+    pthread_mutex_t event_hook_mutex;
+
     struct {
         bool gc_thread_crashed;
         char crash_msg[256];
@@ -338,9 +340,9 @@ rb_mmtk_call_obj_free(MMTk_ObjectReference object)
     struct objspace *objspace = rb_gc_get_objspace();
 
     if (RB_UNLIKELY(rb_gc_event_hook_required_p(RUBY_INTERNAL_EVENT_FREEOBJ))) {
-        rb_gc_worker_thread_set_vm_context(&objspace->vm_context);
+        pthread_mutex_lock(&objspace->event_hook_mutex);
         rb_gc_event_hook(obj, RUBY_INTERNAL_EVENT_FREEOBJ);
-        rb_gc_worker_thread_unset_vm_context(&objspace->vm_context);
+        pthread_mutex_unlock(&objspace->event_hook_mutex);
     }
 
     rb_gc_obj_free(objspace, obj);
@@ -579,6 +581,8 @@ rb_gc_impl_objspace_init(void *objspace_ptr)
     objspace->mutex = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
     objspace->cond_world_stopped = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
     objspace->cond_world_started = (pthread_cond_t)PTHREAD_COND_INITIALIZER;
+
+    objspace->event_hook_mutex = (pthread_mutex_t)PTHREAD_MUTEX_INITIALIZER;
 }
 
 void