perf bench: Add --repeat option

There are a number of benchmarks that do single runs and as a result
does not really help users gain a general idea of how the workload
performs. So the user must either manually do multiple runs or just use
single bogus results.

This option will enable users to specify the amount of runs (arbitrarily
defaulted to 10, to use the existing benchmarks default) through the
'--repeat' option.  Add it to perf-bench instead of implementing it
always in each specific benchmark.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1402942467-10671-2-git-send-email-davidlohr@hp.com


[ Kept the existing default of 10, changing it to something else should
  be done on separate patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>