libc/time: Return GMT instead of NULL for tm_zone

Fixes Node.js which doesn't check the tm_zone member for NULL.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: David Halls <dahalls@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>