Support bootstrap running on old kernel(<5.8)

[linrl3]

# ./bootstrap
libbpf: map 'rb': failed to create: -EINVAL
libbpf: failed to load BPF skeleton 'bootstrap_bpf': -EINVAL
Failed to load and verify BPF skeleton

When I am trying to run bootstrap, I get this error. It's because kernel version lower than 5.8 doesn't support BPF ring buffer map. My kernel version is 5.4.

I wonder if I could create a PR to implement a legacy version of bootstrap(bootstrap-legacy) to let it run on low version kernels. The idea is to use BPF_MAP_TYPE_PERF_EVENT_ARRAY to send data.

[anakryiko]

@linrl3 sure, go ahead. Just mention bootstrap vs bootstrap-legacy differences in the README.md as well

[linrl3]

@anakryiko will do it in the next few days

[linrl3]

@anakryiko Hi Andrii, could you review the PR examples/c: add bootstrap_legacy for compatibility with older kernels #351 ?