perf stat: Don't require a workload when using system wide or CPU options

The "perf stat" command can do system wide counters or one or more cpus.
For these options do not require a workload to be specified.

v2: use perf_target__none per Namhyung's comment.

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/52497F3C.9070908@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>