selftests: net: Add cross-compilation support for BPF programs

The selftests/net does not have proper cross-compilation support, and
does not properly state libbpf as a dependency. Mimic/copy the BPF
build from selftests/bpf, which has the nice side-effect that libbpf
is built as well.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Link: https://lore.kernel.org/r/20221119171841.2014936-1-bjorn@kernel.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>