perf stat: Enable ignore_missing_thread

perf already support ignore_missing_thread for -p, but not yet
applied to `perf stat -p <pid>`. This patch enables ignore_missing_thread
for `perf stat -p <pid>`.

Committer notes:

And here is a refresher about the 'ignore_missing_thread' knob, from a
previous patch using it:

  ca80006 ("perf evsel: Enable ignore_missing_thread for pid option")

  ---
    While monitoring a multithread process with pid option, perf sometimes
    may return sys_perf_event_open failure with 3(No such process) if any of
    the process's threads die before we open the event. However, we want
    perf continue monitoring the remaining threads and do not exit with
    error.
  ---

Signed-off-by: Gang Li <ligang.bdlg@bytedance.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220622030037.15005-1-ligang.bdlg@bytedance.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -2586,6 +2586,8 @@ int cmd_stat(int argc, const char **argv)
 	if (evlist__initialize_ctlfd(evsel_list, stat_config.ctl_fd, stat_config.ctl_fd_ack))
 		goto out;
 
+	/* Enable ignoring missing threads when -p option is defined. */
+	evlist__first(evsel_list)->ignore_missing_thread = target.pid;
 	status = 0;
 	for (run_idx = 0; forever || run_idx < stat_config.run_count; run_idx++) {
 		if (stat_config.run_count != 1 && verbose > 0)