Add _Py_AS_GC macro. It will be used by the trashcan code on object.c.

Include/objimpl.h

@@ -235,10 +235,12 @@ typedef union _gc_head {
 
 extern PyGC_Head _PyGC_generation0;
 
+#define _Py_AS_GC(o) ((PyGC_Head *)(o)-1)
+
 /* Tell the GC to track this object.  NB: While the object is tracked the
  * collector it must be safe to call the ob_traverse method. */
 #define _PyObject_GC_TRACK(o) do { \
-	PyGC_Head *g = (PyGC_Head *)(o)-1; \
+	PyGC_Head *g = _Py_AS_GC(o); \
 	if (g->gc.gc_next != NULL) \
 		Py_FatalError("GC object already in linked list"); \
 	g->gc.gc_next = &_PyGC_generation0; \
@@ -249,7 +251,7 @@ extern PyGC_Head _PyGC_generation0;
 
 /* Tell the GC to stop tracking this object. */
 #define _PyObject_GC_UNTRACK(o) do { \
-	PyGC_Head *g = (PyGC_Head *)(o)-1; \
+	PyGC_Head *g = _Py_AS_GC(o); \
 	g->gc.gc_prev->gc.gc_next = g->gc.gc_next; \
 	g->gc.gc_next->gc.gc_prev = g->gc.gc_prev; \
 	g->gc.gc_next = NULL; \