Use rb_gc_get_ec in rb_gc_event_hook

This would allow rb_gc_event_hook to run in a GC thread that is a
non-Ruby thread.

Author: peterzhu2118

gc.c

@@ -239,7 +239,7 @@ rb_gc_event_hook(VALUE obj, rb_event_flag_t event)
 {
     if (LIKELY(!rb_gc_event_hook_required_p(event))) return;
 
-    rb_execution_context_t *ec = GET_EC();
+    rb_execution_context_t *ec = rb_gc_get_ec();
     if (!ec->cfp) return;
 
     EXEC_EVENT_HOOK(ec, event, ec->cfp->self, 0, 0, 0, obj);

gc/default/default.c

@@ -6415,6 +6415,8 @@ garbage_collect(rb_objspace_t *objspace, unsigned int reason)
 static int
 gc_start(rb_objspace_t *objspace, unsigned int reason)
 {
+    rb_gc_initialize_vm_context(&objspace->vm_context);
+
     unsigned int do_full_mark = !!(reason & GPR_FLAG_FULL_MARK);
 
     if (!rb_darray_size(objspace->heap_pages.sorted)) return TRUE; /* heap is not ready */