selftests/nolibc: link_cross: use /proc/self/cmdline

For CONFIG_NET=n, there would be no /proc/self/net, so, use
/proc/self/cmdline instead.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>