samples: bpf: Add BPF support for xdp_exception tracepoint

This would allow us to store stats for each XDP action, including their
per-CPU counts. Consolidating this here allows all redirect samples to
detect xdp_exception events.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-7-memxor@gmail.com