perf evlist: Add has_user_cpus member

Subsequent patches will need to call perf_evlist__propagate_maps without
reference to a "target".  Add evlist->has_user_cpus to record whether
the user has specified which cpus to target (and therefore whether that
list of cpus should override the default settings for a selected event
i.e. the cpu maps should be propagated)

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Link: http://lkml.kernel.org/r/1441699142-18905-5-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>