perf machine: Introduce synthesize_threads method out of open coded equivalent

Further simplifications to be done on following patch, as most tools
don't use the callback, using instead just the canned
machine__process_event one.

Cc: 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: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-r1m0vuuj3cat4bampno9yc8d@git.kernel.org


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