perf stat: Don't require a workload when using system wide or CPU options

The "perf stat" command can do system wide counters or one or more cpus.
For these options do not require a workload to be specified.

v2: use perf_target__none per Namhyung's comment.
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/52497F3C.9070908@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -1659,8 +1659,9 @@ int cmd_stat(int argc, const char **argv, const char *prefix __maybe_unused)
 	} else if (big_num_opt == 0) /* User passed --no-big-num */
 		big_num = false;
 
-	if (!argc && !perf_target__has_task(&target))
+	if (!argc && perf_target__none(&target))
 		usage_with_options(stat_usage, options);
+
 	if (run_count < 0) {
 		usage_with_options(stat_usage, options);
 	} else if (run_count == 0) {