bpo-36710: Add runtime variable to Py_InitializeEx() (GH-12939)

Py_InitializeEx() now uses a runtime variable passed to subfunctions,
rather than working directly on the global variable _PyRuntime.

Add 'runtime' parameter to _PyCoreConfig_Write(), _PySys_Create(),
_PySys_InitMain(), _PyGILState_Init(),
emit_stderr_warning_for_legacy_locale() and other subfunctions.