gh-123923: Defer refcounting for f_executable in _PyInterpreterFrame (#123924)

Use a `_PyStackRef` and defer the reference to `f_executable` when
possible. This avoids some reference count contention in the common case
of executing the same code object from multiple threads concurrently in
the free-threaded build.

Python/gc.c

@@ -534,6 +534,13 @@ visit_decref(PyObject *op, void *parent)
     return 0;
 }
 
+int
+_PyGC_VisitStackRef(_PyStackRef *ref, visitproc visit, void *arg)
+{
+    Py_VISIT(PyStackRef_AsPyObjectBorrow(*ref));
+    return 0;
+}
+
 int
 _PyGC_VisitFrameStack(_PyInterpreterFrame *frame, visitproc visit, void *arg)
 {