perf machine: Simplify synthesize_threads method

perf machine: Simplify synthesize_threads method

Several tools (top, kvm) don't need to be called back to process each of
the syntheiszed records, instead relying on the machine__process_event
function to change the per machine data structures that represent
threads and mmaps, so provide a way to ask for this common idiom.

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-pusqibp8n3c4ynegd1frn4zd@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>