perf stat: Support regex pattern in --for-each-cgroup

To make the command line even more compact with cgroups, support regex
pattern matching in cgroup names.

  $ perf stat -a -e cpu-clock,cycles --for-each-cgroup ^foo sleep 1

          3,000.73 msec cpu-clock                 foo #    2.998 CPUs utilized
    12,530,992,699      cycles                    foo #    7.517 GHz                      (100.00%)
          1,000.61 msec cpu-clock                 foo/bar #    1.000 CPUs utilized
     4,178,529,579      cycles                    foo/bar #    2.506 GHz                      (100.00%)
          1,000.03 msec cpu-clock                 foo/baz #    0.999 CPUs utilized
     4,176,104,315      cycles                    foo/baz #    2.505 GHz                      (100.00%)

       1.000892614 seconds time elapsed

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20201027072855.655449-2-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>