tools/nolibc: add namespace functionality

tools/nolibc: add namespace functionality

This is used in various selftests and will be handy when integrating
those with nolibc.

Not all configurations support namespaces, so skip the tests where
necessary.  Also if the tests are running without privileges.
Enable the namespace configuration for those architectures where it is not
enabled by default.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250428-nolibc-misc-v2-12-3c043eeab06c@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>