Proper MB_CUR_MAX.

Previously this was hard coded to 4. This is only the case for UTF-8
locales.

As a side effect, this properly reports C.UTF-8 as the default locale
instead of C.

Change-Id: I7c73cc8fe6ffac61d211cd5f75287e36de06f4fc
(cherry picked from commit 1aec7c1a35)

libc/include/stdlib.h

@@ -161,7 +161,8 @@ extern int      mbtowc(wchar_t *, const char *, size_t);
 extern int	wctomb(char *, wchar_t);
 extern size_t	wcstombs(char *, const wchar_t *, size_t);
 
-#define MB_CUR_MAX 4U
+extern size_t __mb_cur_max(void);
+#define MB_CUR_MAX __mb_cur_max()
 
 __END_DECLS