selftests/bpf: Switch perf_buffer test to tracepoint and skeleton

Switch perf_buffer test to use skeleton to avoid use of bpf_prog_load() and
make test a bit more succinct. Also switch BPF program to use tracepoint
instead of kprobe, as that allows to support older kernels, which had
tracepoint support before kprobe support in the form that libbpf expects
(i.e., libbpf expects /sys/bus/event_source/devices/kprobe/type, which doesn't
always exist on old kernels).

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200708015318.3827358-7-andriin@fb.com