perf events: Default to using event__process_lost

Tool developers have to fill in a 'perf_event_ops' method table to
specify how to handle each event, so far the ones that were not
explicitely especified would get a stub that would just discard the
event.

Change that so that tool developers can get the lost event details and
the total number of such events at the end of 'perf report -D' output.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>