selftests/landlock: Make tests build with old libc

Replace SYS_<syscall> with __NR_<syscall>.  Using the __NR_<syscall>
notation, provided by UAPI, is useful to build tests on systems without
the SYS_<syscall> definitions.

Replace SYS_pivot_root with __NR_pivot_root, and SYS_move_mount with
__NR_move_mount.

Define renameat2() and RENAME_EXCHANGE if they are unknown to old build
systems.

Cc: Shuah Khan <shuah@kernel.org>
Link: https://lore.kernel.org/r/20220506160820.524344-3-mic@digikod.net


Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>