perf tool_pmu: Make core_wide and target_cpu json events

perf tool_pmu: Make core_wide and target_cpu json events

For the sake of better documentation, add core_wide and target_cpu to
the tool.json. When the values of system_wide and
user_requested_cpu_list are unknown, use the values from the global
stat_config.

Example output showing how '-a' modifies the values in `perf stat`:
```
$ perf stat -e core_wide,target_cpu true

 Performance counter stats for 'true':

                 0      core_wide
                 0      target_cpu

       0.000993787 seconds time elapsed

       0.001128000 seconds user
       0.000000000 seconds sys

$ perf stat -e core_wide,target_cpu -a true

 Performance counter stats for 'system wide':

                 1      core_wide
                 1      target_cpu

       0.002271723 seconds time elapsed

$ perf list
...
tool:
  core_wide
       [1 if not SMT,if SMT are events being gathered on all SMT threads 1 otherwise 0. Unit: tool]
...
  target_cpu
       [1 if CPUs being analyzed,0 if threads/processes. Unit: tool]
...
```

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>