perf stat: Add interval printing

This patch adds a new printing mode for perf stat.  It allows interval
printing. That means perf stat can now print event deltas at regular
time interval.  This is useful to detect phases in programs.

The -I option enables interval printing. It expects an interval duration
in milliseconds. Minimum is 100ms. Once, activated perf stat prints
events deltas since last printout. All modes are supported.

$ perf stat -I 1000 -e cycles noploop 10
noploop for 10 seconds
 #           time             counts events
      1.000109853      2,388,560,546 cycles
      2.000262846      2,393,332,358 cycles
      3.000354131      2,393,176,537 cycles
      4.000439503      2,393,203,790 cycles
      5.000527075      2,393,167,675 cycles
      6.000609052      2,393,203,670 cycles
      7.000691082      2,393,175,678 cycles

The output format makes it easy to feed into a plotting program such as
gnuplot when the -I option is used in combination with the -x option:

$ perf stat -x, -I 1000 -e cycles noploop 10
noploop for 10 seconds
1.000084113,2378775498,cycles
2.000245798,2391056897,cycles
3.000354445,2392089414,cycles
4.000459115,2390936603,cycles
5.000565341,2392108173,cycles

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1359460064-3060-3-git-send-email-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>