perf util: Move bpf_perf definitions to a libperf header

perf util: Move bpf_perf definitions to a libperf header

By following the same protocol, other tools can share hardware PMCs with
perf. Move perf_event_attr_map_entry and BPF_PERF_DEFAULT_ATTR_MAP_PATH to
bpf_perf.h for other tools to use.

Signed-off-by: Song Liu <song@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: kernel-team@fb.com
Link: https://lore.kernel.org/r/20210425214333.1090950-2-song@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>