perf top: Default to system wide using perf_target methods

perf top: Default to system wide using perf_target methods

Additionally we were not checking if a cpu list had been provided by the
user. Fix that.

Reported-by: David Ahern <dsahern@gmail.com>
Reported-by: Namhyung Kim <namhyung@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.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-ao3zrouylwmt7h9ikj0krubi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>