tools/nolibc: add support for uname(2)

All supported kernels are assumed to use struct new_utsname.
This is validated in test_uname().

uname(2) can for example be used in ksft_min_kernel_version() from the
kernels selftest framework.

Link: https://lore.kernel.org/lkml/20240412123536.GA32444@redhat.com/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>