kselftest/seccomp: Use kselftest output functions for benchmark

In preparation for trying to output the test results themselves in TAP
format rework all the prints in the benchmark to use the kselftest output
functions. The uses of system() all produce single line output so we can
avoid having to deal with fully managing the child process and continue to
use system() by simply printing an empty message before we invoke system().
We also leave one printf() used to complete a line of output in place.

Tested-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>