perf stat: Add support to print counts for fixed times

[ufo19890607]

From: yuzhoujian <yuzhoujian@xxxxxxxxxxxxxxx>

Hi Jirka
And how can you control the output for perf stat, if I don't want to use the 
"sleep" workload, like some user programs. I want to check the "cycles"
for this program when it just begin to run.

root@node10:/home$ gcc -g -o malloc malloc.c
root@node10:/home$ ./perf stat -e cycles -I 1000 --times-print 3 ./malloc &
[1] 32586
#           time             counts unit events
     1.000094153      2,914,162,979      cycles                                                      
     2.000323667      3,062,832,799      cycles                                                      
     3.000443807      3,063,146,609      cycles

Best wishes