Fix BDW ephemeron implementation

* src/gc-ephemeron.c: Use key null-ness as dead ephemeron indicator;
works better with BDW-GC's disappearing link.
* src/bdw.c (gc_heap_ephemeron_trace_epoch): Fix to actually define the
epoch.  Whoops!

src/bdw.c

@@ -181,7 +181,7 @@ struct gc_ephemeron* gc_allocate_ephemeron(struct gc_mutator *mut) {
 }
 
 unsigned gc_heap_ephemeron_trace_epoch(struct gc_heap *heap) {
-  return 0;
+  return GC_get_gc_no();
 }
 
 void gc_ephemeron_init(struct gc_mutator *mut, struct gc_ephemeron *ephemeron,