linux config: add support for xdp sockets and ebpf jit

xdp socket support (AF_XDP) is the new way of implementing high
performance networking on linux. on arch linux and debian this is
already enabled (checked via the links from the nixos manual).

moreover, these flags are suggested by the bpf documentation at cilium:

https://cilium.readthedocs.io/en/latest/bpf/#compiling-the-kernel

additionally the flag `BPF_JIT_ALWAYS_ON` on was suggested to help
spectre attack mitigations:

https://github.com/torvalds/linux/commit/290af86629b25ffd1ed6232c4e9107da031705cb