perf cpu_map: Add cpu_map__empty_new function

Adding cpu_map__empty_new interface to create empty cpumap with given
size. The cpumap entries are initialized with -1.

It'll be used for caching cpu_map in following patches.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1445784728-21732-2-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>