tools/libpf: Add offsetof/container_of macro in bpf_helpers.h

These two helpers will be used later in bpf_iter bpf program
bpf_iter_netlink.c. Put them in bpf_helpers.h since they could
be useful in other cases.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175919.2477104-1-yhs@fb.com