bpo-34523: Fix C locale coercion on FreeBSD CURRENT (GH-10672) (GH-10673)

bpo-34523, bpo-35290: C locale coercion now resets the Python
internal "force ASCII" mode. This change fix the filesystem encoding
on FreeBSD CURRENT, which has a new "C.UTF-8" locale, when
the UTF-8 mode is disabled.

Add _Py_ResetForceASCII(): _Py_SetLocaleFromEnv() now calls it.

(cherry picked from commit 353933e712b6c7f7ba9a9a50bd5bd472db7c35d0)
diff --git a/Python/fileutils.c b/Python/fileutils.c
index b77e490..5e71d37 100644
--- a/Python/fileutils.c
+++ b/Python/fileutils.c
@@ -191,6 +191,12 @@
 }
 
 
+void
+_Py_ResetForceASCII(void)
+{
+    force_ascii = -1;
+}
+
 
 static int
 encode_ascii(const wchar_t *text, char **str,
@@ -252,6 +258,12 @@
 {
     return 0;
 }
+
+void
+_Py_ResetForceASCII(void)
+{
+    /* nothing to do */
+}
 #endif   /* !defined(__APPLE__) && !defined(__ANDROID__) && !defined(MS_WINDOWS) */