ebpf: compile with clang -target bpf

ebpf: compile with clang -target bpf

Enable compiling eBPF programs with clang -target bpf.

This is mostly to workaround a bug in libbpf, where clang > ver 4.0.0
generates some ELF sections (.eh_frame) when -target bpf is NOT specified,
and libbpf fails loading such files.

Notice libbpf is provided by the kernel, and in kernel v4.16 the library
will contain the needed function for attaching to the XDP hook.

Kernel commit 949abbe88436 ("libbpf: add function to setup XDP")
 https://git.kernel.org/torvalds/c/949abbe88436

The library fix has reached kernel v4.16 but the workaround for Suricata
is interesting anyway in case people use a kernel v4.15.

Signed-off-by: Jesper Dangaard Brouer <netoptimizer@brouer.com>