perf record: Add -a as default target

Running 'perf record' with no target (-a, -p, -t, etc) will now collect
system wide data.

Commiter notes:

Testing it:

  [root@jouet ~]# perf record
  ^C[ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 1.351 MB perf.data (366 samples) ]
  #

is equivalent to:

  # perf record -a
  ^C[ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 1.411 MB perf.data (978 samples) ]
  #

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
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/20170217170018.GA15389@krava


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