To improve 'perf diff', implement a --cpu filter option.

Multiple CPUs can be provided as a comma-separated list with no space:
0,1.  Ranges of CPUs are specified with -: 0-2. Default is to report
samples on all CPUs.

For example,

  perf diff --cpu 0,1

It only diff the samples for CPU0 and CPU1.
Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1551791143-10334-3-git-send-email-yao.jin@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>