tools/nolibc: crt: mark _start_c() as used

During LTO the reference from the asm startup code to the _start_c()
function is not visible and _start_c() is removed.
This will then lead to errors during linking.
As _start_c() is indeed always used, mark it as such.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240812-nolibc-lto-v2-1-736af7bbefa8@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>