perf report: Introduce helpers for processing callchains

Continuing to try to remove the code duplication introduced with mem and
branch hist entry code, this time providing prologue and epilogues to
deal with callchains when processing samples.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-js3pour59yk2aibqzb1tpumh@git.kernel.org


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