Clean up some code

- Use good ELF technique in cosmo_dlopen()
- Make strerror() conform more to other libc impls
- Introduce __clear_cache() and use it in cosmo_dlopen()
- Remove libc/fmt/fmt.h header (trying to kill off LIBC_FMT)

libc/intrin/strsignal.c

@@ -38,5 +38,5 @@ static char g_strsignal[21];
  * @threadunsafe
  */
 char *strsignal(int sig) {
-  return strsignal_r(sig, g_strsignal);
+  return (char *)strsignal_r(sig, g_strsignal);
 }