perf python: Add evlist all_cpus accessor

Add a means to get the reference counted all_cpus CPU map from an
evlist in its python form.

Reviewed-by: Howard Chu <howardchu95@gmail.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20250228222308.626803-10-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>