perf tools: Introduce trim function

To be used in cases for both sides trim.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andreas Hollmann <hollmann@in.tum.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1460013073-18444-1-git-send-email-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>