Introduce 'b' modifier to event parser, which means use BPF program to
manage this event. This is the same as --bpf-counters option, but only
applies to this event. For example,

  perf stat -e cycles:b,cs               # use bpf for cycles, but not cs
  perf stat -e cycles,cs --bpf-counters  # use bpf for both cycles and cs
Suggested-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20210425214333.1090950-5-song@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>