perf script: Add process_stat/process_stat_interval scripting interface

perf script: Add process_stat/process_stat_interval scripting interface

Python and perl scripting code will define those callbacks and get stat
data.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452028152-26762-5-git-send-email-jolsa@kernel.org
[ Rename 'time' parameters to 'tstamp', to fix the build in older distros ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>