seccomp: Add some basic selftests

Add basic selftests of secure computing mode, both STRICT and
FILTER.

Signed-off-by: David Drysdale <drysdale@google.com>
seccomp,um: Enable seccomp in um/x86

Signed-off-by: Meredydd Luff <meredydd@chromium.org>
Signed-off-by: David Drysdale <drysdale@google.com>