bpo-34403: On HP-UX, force ASCII for C locale (GH-8969)

On HP-UX with C or POSIX locale, sys.getfilesystemencoding() now returns
"ascii" instead of "roman8" (when the UTF-8 Mode is disabled and the C locale
is not coerced).

nl_langinfo(CODESET) announces "roman8" whereas it uses the Latin1
encoding in practice.
diff --git a/Python/coreconfig.c b/Python/coreconfig.c
index acf4645..d08d75b 100644
--- a/Python/coreconfig.c
+++ b/Python/coreconfig.c
@@ -828,18 +828,21 @@
 static void
 config_init_locale(_PyCoreConfig *config)
 {
-    if (_Py_LegacyLocaleDetected()) {
+    if (config->coerce_c_locale < 0) {
         /* The C locale enables the C locale coercion (PEP 538) */
-        if (config->coerce_c_locale < 0) {
+        if (_Py_LegacyLocaleDetected()) {
             config->coerce_c_locale = 1;
         }
     }
+
 #ifndef MS_WINDOWS
-    const char *ctype_loc = setlocale(LC_CTYPE, NULL);
-    if (ctype_loc != NULL
-       && (strcmp(ctype_loc, "C") == 0 || strcmp(ctype_loc, "POSIX") == 0)) {
+    if (config->utf8_mode < 0) {
         /* The C locale and the POSIX locale enable the UTF-8 Mode (PEP 540) */
-        if (config->utf8_mode < 0) {
+        const char *ctype_loc = setlocale(LC_CTYPE, NULL);
+        if (ctype_loc != NULL
+           && (strcmp(ctype_loc, "C") == 0
+               || strcmp(ctype_loc, "POSIX") == 0))
+        {
             config->utf8_mode = 1;
         }
     }