perf tools: Identify which comms are from exec

For grouping together all the data from a single execution, which is
needed for pairing calls and returns e.g. any outstanding calls when a
process exec's will never return.

Signed-off-by: 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: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1406786474-9306-2-git-send-email-adrian.hunter@intel.com
[ Remove testing if comm->exec is false before setting it to true ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>