Add _PyObject_FastCallVa() helper

Issue #28915: Add _PyObject_FastCallVa() helper to factorize code of functions:

* PyObject_CallFunctionObjArgs()
* PyObject_CallMethodObjArgs()
* _PyObject_CallMethodIdObjArgs()

Inline objargs_mkstack() into _PyObject_FastCallVa(), remove
objargs_mkstack().