perf tools: Generalize event synthesizing routines

Because we will need it in 'perf top' to support userspace
symbols for existing threads.

Now we pass a callback that will receive the synthesized event
and then write it to the output file in 'perf record' and in the
upcoming patch for 'perf top' we will just immediatelly create
the in memory representation of threads and maps.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1256592199-9608-2-git-send-email-acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>