selftests/bpf: Add mprog API tests for BPF tcx links

Add a big batch of test coverage to assert all aspects of the tcx link API:

  # ./vmtest.sh -- ./test_progs -t tc_links
  [...]
  #224     tc_links_after:OK
  #225     tc_links_append:OK
  #226     tc_links_basic:OK
  #227     tc_links_before:OK
  #228     tc_links_both:OK
  #229     tc_links_chain_classic:OK
  #230     tc_links_dev_cleanup:OK
  #231     tc_links_first:OK
  #232     tc_links_invalid:OK
  #233     tc_links_last:OK
  #234     tc_links_prepend:OK
  #235     tc_links_replace:OK
  #236     tc_links_revision:OK
  Summary: 13/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>