libroot: Do not return LC_GLOBAL_LOCALE from __current_locale_t().

Not all things that accept locale_t can accept LC_GLOBAL_LOCALE,
specifically nl_langinfo_l does not (the specification indicates
that passing it is undefined behavior; indeed, no C library I glanced
at seems to support that.)

So, return a static locale_t pointing to the global values instead.

Fixes #17875 and #17876.

src/system/libroot/posix/locale/locale_t.cpp

@@ -266,7 +266,12 @@ extern "C" locale_t
 __current_locale_t()
 {
 	locale_t locale = (locale_t)GetCurrentLocaleInfo();
-	if (locale == NULL)
-		locale = LC_GLOBAL_LOCALE;
+	if (locale == NULL) {
+		static LocaleBackendData global_locale_t;
+		global_locale_t.backend = gGlobalLocaleBackend;
+		global_locale_t.databridge = &gGlobalLocaleDataBridge;
+		return (locale_t)&global_locale_t;
+	}
+
 	return locale;
 }