perf/benchmark: add a new benchmark for seccom_unotify

perf/benchmark: add a new benchmark for seccom_unotify

The benchmark is similar to the pipe benchmark. It creates two processes,
one is calling syscalls, and another process is handling them via seccomp
user notifications. It measures the time required to run a specified number
of interations.

 $ ./perf bench sched  seccomp-notify --sync-mode --loop 1000000
 # Running 'sched/seccomp-notify' benchmark:
 # Executed 1000000 system calls

     Total time: 2.769 [sec]

       2.769629 usecs/op
         361059 ops/sec

 $ ./perf bench sched  seccomp-notify
 # Running 'sched/seccomp-notify' benchmark:
 # Executed 1000000 system calls

     Total time: 8.571 [sec]

       8.571119 usecs/op
         116670 ops/sec

Signed-off-by: Andrei Vagin <avagin@google.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230308073201.3102738-7-avagin@google.com
Link: https://lore.kernel.org/r/20230630051953.454638-1-avagin@gmail.com
[kees: Added PRIu64 format string]
Signed-off-by: Kees Cook <keescook@chromium.org>