selftests/seccomp: Change the syscall used in KILL_THREAD test

The Bionic version of pthread_create used on Android calls the prctl
function to give the stack and thread local storage a useful name. This
will cause the KILL_THREAD test to fail as it will kill the thread as
soon as it is created.

change the test to use getpid instead of prctl.

Signed-off-by: Terry Tritton <terry.tritton@linaro.org>
Link: https://lore.kernel.org/r/20240124141357.1243457-3-terry.tritton@linaro.org


Signed-off-by: Kees Cook <keescook@chromium.org>