tools/nolibc: move uname() and friends to sys/utsname.h

This is the location regular userspace expects these definitions.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250515-nolibc-sys-v1-7-74f82eea3b59@weissschuh.net