selftests/nolibc: Fix build with host headers and libc

selftests/nolibc: Fix build with host headers and libc

Many systems don't have strlcpy() or strlcat() and readdir_r() is
deprecated. This makes the tests fail to build with the host headers.
Disable the 'directories' test and define strlcpy(), strlcat() and
readdir_r() using #defines so that the code compiles.

Fixes: 6fe8360b16acb ("selftests/nolibc: also test libc-test through regular selftest framework")
Signed-off-by: David Laight <david.laight.linux@gmail.com>
Link: https://patch.msgid.link/20260223101735.2922-4-david.laight.linux@gmail.com
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>