perf tools: Allow handling a NULL cpu_map as meaning "all cpus"

Or one with cpu_map->map[0] == -1.

Reducing the boilerplate in setting up an evlist by nor requiring a
cpu_map to be created at all.

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: Namhyung Kim <namhyung@gmail.com>
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-rnaqn3dtnsfo1wlbbf3fhx00@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>