bpo-34485: Fix _Py_InitializeCore() for C locale coercion (GH-8979)

* _Py_InitializeCore() now sets the LC_CTYPE locale to the user
  preferred locale before checking if the C locale should be coerced
  or not in _PyCoreConfig_Read().
* Fix pymain_read_conf(): remember if the C locale has been coerced
  when the configuration should be read again if the encoding has
  changed.