perf callchain: Abstract callchain print function

This is a preparation to support for printing other type of callchain
value like count or period.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1447047946-1691-4-git-send-email-namhyung@kernel.org


[ renamed new _sprintf_ operation to _scnprintf_ ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>