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/pylifecycle.c b/Python/pylifecycle.c
index 7cf4a6d..dec8904 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -287,7 +287,7 @@
 static void
 _emit_stderr_warning_for_legacy_locale(const _PyCoreConfig *core_config)
 {
-    if (core_config->coerce_c_locale_warn && _Py_LegacyLocaleDetected()) {
+    if (core_config->preconfig.coerce_c_locale_warn && _Py_LegacyLocaleDetected()) {
         PySys_FormatStderr("%s", _C_LOCALE_WARNING);
     }
 }