perf script: look up thread using tid instead of pid

perf script: look up thread using tid instead of pid

This allows the thread name to be dispalyed when dumping
events:
           myapp 25118 [000] 450385.538815: context-switches ...
    myapp:worker 25119 [000] 450385.538894: context-switches ...

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1324578603-12762-4-git-send-email-dsahern@gmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>