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 <bp@alien8.de>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo 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@krava


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>