perf tools: Look up thread names for system wide profiling

This handles multithreaded processes with named threads when doing
system wide profiling: the comm for each thread is looked up allowing
them to be different from the thread group leader.

v2:
- fixed sizeof arg to perf_event__get_comm_tgid

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1324578603-12762-3-git-send-email-dsahern@gmail.com


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