bs-ebpf: Make it work on rust's Stable channel

[oskardrums]

The only reason bs-ebpf is currently nighlty-only is because we use the syscall crate to call create a raw call to bpf(2), and the syscall crate uses #[feature(asm)], and that's marked as unstable for the foreseeable future...

We only use syscall in this line:

bs/bs-ebpf/src/lib.rs

Line 171 in 976c780

let fd = unsafe { syscall!(BPF, 5, ptr, size_of_val(&attr)) as i32 };

So we probably need some build.rs script to generate a .a static library that exposes just a bpf(2) wrapper 🤷‍♂️