perf tools: Force uncore events to system wide monitoring

Make system wide (-a) the default option if no target was specified and
one of following conditions is met:

  - there's no workload specified (current behaviour)
  - there is workload specified but all requested
    events are system wide ones

Mixed events core/uncore with workload:

  $ perf stat -e 'uncore_cbox_0/clockticks/,cycles' sleep 1

   Performance counter stats for 'sleep 1':

     <not supported>      uncore_cbox_0/clockticks/
             980,489      cycles

         1.000897406 seconds time elapsed

Uncore event with workload:

  $ perf stat -e 'uncore_cbox_0/clockticks/' sleep 1

   Performance counter stats for 'system wide':

  281,473,897,192,670      uncore_cbox_0/clockticks/

         1.000833784 seconds time elapsed

Committer note:

When testing I realized the default case for !root, i.e. no events
passed via -e, was broke by v2 of this patch, reported and after a
patch provided by Jiri it is back working:

  [acme@jouet linux]$ perf stat usleep 1

   Performance counter stats for 'usleep 1':

         0.401335      task-clock:u (msec)     #   0.297 CPUs utilized
                0      context-switches:u      #   0.000 K/sec
                0      cpu-migrations:u        #   0.000 K/sec
               48      page-faults:u           #   0.120 M/sec
          458,146      cycles:u                #   1.142 GHz
          245,113      instructions:u          #   0.54  insn per cycle
           47,991      branches:u              # 119.578 M/sec
            4,022      branch-misses:u         #   8.38% of all branches

      0.001350029 seconds time elapsed

  [acme@jouet linux]$

Suggested-and-Tested-by: Borislav Petkov <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/20170227094818.GA12764@krava
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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