Support and test BTF output

[jpsamaroo]

BPF Type Format (BTF) is the debuginfo for eBPF, and is used by libbpf to ensure that accesses to in-kernel structures are correct and safe to execute, and to perform relocations and rewrites as necessary. We should test that our BTF output is solid and matches what libbpf expects.

[jpsamaroo]

We could also help ourselves in the interim by dynamically generating kernel structures dynamically, as an optional build-time step, by running bpftool btf dump file /sys/kernel/btf/vmlinux format c when possible (as pointed out by https://facebookmicrosites.github.io/bpf/blog/2020/02/19/bpf-portability-and-co-re.html).