perf tool_pmu: Allow num_cpus(_online) to be specific to a cpumask

perf tool_pmu: Allow num_cpus(_online) to be specific to a cpumask

For hybrid metrics it is useful to know the number of p-core or e-core
CPUs. If a cpumask is specified for the num_cpus or num_cpus_online
tool events, compute the value relative to the given mask rather than
for the full system.

```
$ sudo /tmp/perf/perf stat -e 'tool/num_cpus/,tool/num_cpus,cpu=cpu_core/,
  tool/num_cpus,cpu=cpu_atom/,tool/num_cpus_online/,tool/num_cpus_online,
  cpu=cpu_core/,tool/num_cpus_online,cpu=cpu_atom/' true

 Performance counter stats for 'true':

                28      tool/num_cpus/
                16      tool/num_cpus,cpu=cpu_core/
                12      tool/num_cpus,cpu=cpu_atom/
                28      tool/num_cpus_online/
                16      tool/num_cpus_online,cpu=cpu_core/
                12      tool/num_cpus_online,cpu=cpu_atom/

       0.000767205 seconds time elapsed

       0.000938000 seconds user
       0.000000000 seconds sys
```

Reviewed-by: Thomas Falcon <thomas.falcon@intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250719030517.1990983-6-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>