libbpf: Add cscope and tags targets to Makefile

Using cscope and/or TAGS files for navigating the source code is useful.
Add simple targets to the Makefile to generate the index files for both
tools.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191004153444.1711278-1-toke@redhat.com