bpo-36710: Add runtime variable to Py_FinalizeEx() (GH-12937)

* Add a 'runtime' variable to Py_FinalizeEx() rather than working
  directly on the global variable _PyRuntime
* Add a 'runtime' parameter to _PyGC_Fini(), _PyGILState_Fini()
  and call_ll_exitfuncs()