perf stat: Add -a as default target

Boris asked for default -a option in case we monitor only uncore events.

While implementing that I thought it might be actually useful to make it
overall default.

Running 'perf stat' will now collect system wide data.

Committer note:

Testing it:

  # perf stat
  ^C
   Performance counter stats for 'system wide':

         3571.559178      cpu-clock (msec)          #    4.000 CPUs utilized
               3,346      context-switches          #    0.937 K/sec
                 277      cpu-migrations            #    0.078 K/sec
              57,271      page-faults               #    0.016 M/sec
       4,535,633,835      cycles                    #    1.270 GHz
       6,389,736,516      instructions              #    1.41  insn per cycle
       1,541,293,875      branches                  #  431.547 M/sec
          14,526,396      branch-misses             #    0.94% of all branches

         0.892950118 seconds time elapsed

  #
Requested-and-Acked-by: NBorislav Petkov <bp@alien8.de>
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20170217170034.GB15389@kravaSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -63,7 +63,7 @@ report::
 
 -a::
 --all-cpus::
-        system-wide collection from all CPUs
+        system-wide collection from all CPUs (default if no target is specified)
 
 -c::
 --scale::

tools/perf/builtin-stat.c

@@ -2445,8 +2445,9 @@ 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;
 
+	/* Make system wide (-a) the default target. */
 	if (!argc && target__none(&target))
-		usage_with_options(stat_usage, stat_options);
+		target.system_wide = true;
 
 	if (run_count < 0) {
 		pr_err("Run count must be a positive number\n");