perf trace: Explicitly enable system-wide mode if no option is given

When no target cpu/user/task option is given, perf trace will do its job
system wide for all online cpus.  Make it explicit to reduce possible
confusion when reading code.

No functional changes intended.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1349413336-26936-3-git-send-email-namhyung@kernel.org


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