perf stat: Add feature to run and measure a command multiple times

Add the --repeat <n> feature to perf stat, which repeats a given
command up to a 100 times, collects the stats and calculates an
average and a stddev.

For example, the following oneliner 'perf stat' command runs hackbench
5 times and prints a tabulated result of all metrics, with averages
and noise levels (in percentage) printed:

 aldebaran:~/linux/linux/tools/perf> ./perf stat --repeat 5 ~/hackbench 10
 Time: 0.117
 Time: 0.108
 Time: 0.089
 Time: 0.088
 Time: 0.100

 Performance counter stats for '/home/mingo/hackbench 10' (5 runs):

    1243.989586  task-clock-msecs     #     10.460 CPUs    ( +-   4.720% )
          47706  context-switches     #      0.038 M/sec   ( +-  19.706% )
            387  CPU-migrations       #      0.000 M/sec   ( +-   3.608% )
          17793  page-faults          #      0.014 M/sec   ( +-   0.354% )
     3770941606  cycles               #   3031.329 M/sec   ( +-   4.621% )
     1566372416  instructions         #      0.415 IPC     ( +-   2.703% )
       16783421  cache-references     #     13.492 M/sec   ( +-   5.202% )
        7128590  cache-misses         #      5.730 M/sec   ( +-   7.420% )

    0.118924455  seconds time elapsed.

The goal of this feature is to allow the reliance on these accurate
statistics and to know how many times a command has to be repeated
for the noise to go down to an acceptable level.

(The -v option can be used to see a line printed out as each run progresses.)

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>