selftests/bpf: Add fexit and kretprobe triggering benchmarks

We already have kprobe and fentry benchmarks. Let's add kretprobe and
fexit ones for completeness.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240309005124.3004446-1-andrii@kernel.org