perf bpf: Decouple creating the evlist from adding the SB event

Renaming bpf_event__add_sb_event() to evlist__add_sb_event() and
requiring that the evlist be allocated beforehand.

This will allow using the same side band thread and evlist to be used
for multiple purposes in addition to react to PERF_RECORD_BPF_EVENT soon
after they are generated.

Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: http://lore.kernel.org/lkml/20200429131106.27974-4-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>