selftests/seccomp: fix 32-bit build warnings

The casting was done incorrectly for 32-bit builds. Fixed to use uintptr_t.

Reported-by: Eric Adams <adamse@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>