selftests/bpf: Add kprobe multi triggering benchmarks

Adding kprobe multi triggering benchmarks. It's useful now to bench
new fprobe implementation and might be useful later as well.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240311211023.590321-1-jolsa@kernel.org