BPF programs are useful in perf to profile BPF programs.

BPF skeleton is by far the easiest way to write BPF tools. Enable
building BPF skeletons in util/bpf_skel. A dummy bpf skeleton is added.
More bpf skeletons will be added for different use cases.
Signed-off-by: Song Liu <songliubraving@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-team@fb.com
Link: http://lore.kernel.org/lkml/20201229214214.3413833-3-songliubraving@fb.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>