tools/nolibc: stackprotector.h: make __stack_chk_init static

This allows to generate smaller text/data/dec size.

As the _start_c() function added by crt.h, __stack_chk_init() is called
from _start_c() instead of the assembly _start. So, it is able to mark
it with static now.

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