bpo-36142: Add _PyPreConfig.utf8_mode (GH-12174)

* Move following fields from _PyCoreConfig to _PyPreConfig:

  * coerce_c_locale
  * coerce_c_locale_warn
  * legacy_windows_stdio
  * utf8_mode

* _PyPreConfig_ReadFromArgv() is now responsible to choose the
  filesystem encoding
* _PyPreConfig_Write() now sets the LC_CTYPE locale
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 4b12280..50ba1a7 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2181,7 +2181,7 @@
     SetFlag(config->use_hash_seed == 0 || config->hash_seed != 0);
     SetFlag(config->preconfig.isolated);
     PyStructSequence_SET_ITEM(seq, pos++, PyBool_FromLong(config->dev_mode));
-    SetFlag(config->utf8_mode);
+    SetFlag(config->preconfig.utf8_mode);
 #undef SetFlag
 
     if (PyErr_Occurred()) {