Check if neither of --pid, --tid or --uid was specified and if so, set
system_wide appropriately.

Namhyung's patch would make using any of the above target specifiers
emit a warning in perf_target__validate, since it would see
target.system_wide set and one of the others as well.

So set system_wide after validation.
Suggested-by: David Ahern <dsahern@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-6e4zrji1uw0rinfyoitl0wi4@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>