perf stat: Display length strings of each run for --table option

Adding support to display visual aid 'length strings' to easily spot the
biggest difference in time table.

  $ perf stat -r 10 --table perf bench sched pipe
  ...

   Performance counter stats for './perf bench sched pipe' (5 runs):

             # Table of individual measurements:
             5.189 (-0.293) #
             5.189 (-0.294) #
             5.186 (-0.296) #
             5.663 (+0.181) ##
             6.186 (+0.703) ####

             # Final result:
             5.483 +- 0.198 seconds time elapsed  ( +-  3.62% )

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180423090823.32309-9-jolsa@kernel.org
[ Updated 'perf stat --table' man page entry ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-stat.txt

@@ -123,14 +123,14 @@ spreadsheets. Columns are separated by the string specified in SEP.
    Performance counter stats for 'perf bench sched pipe' (5 runs):
 
              # Table of individual measurements:
-             5.379 (-0.176)
-             5.243 (-0.311)
-             5.238 (-0.317)
-             5.536 (-0.019)
-             6.377 (+0.823)
+             5.189 (-0.293) #
+             5.189 (-0.294) #
+             5.186 (-0.296) #
+             5.663 (+0.181) ##
+             6.186 (+0.703) ####
 
              # Final result:
-             5.555 +- 0.213 seconds time elapsed  ( +-  3.83% )
+             5.483 +- 0.198 seconds time elapsed  ( +-  3.62% )
 
 -G name::
 --cgroup name::