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: Borislav Petkov <[e-mail]>
Signed-off-by: Jiri Olsa <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/20170217170034.GB15389@krava
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-stat.txt

···
 
 -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

···
 	} 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");