build,test: fix build-addons dependency chain

* Make the 'extract embedded addons in the documentations' step a normal
  prerequisite.  As an order-only prerequisite, it's sometimes skipped
  when it shouldn't be.

* Make `tools/doc/addon-verify.js` a dependency of that step.  Changes
  to that file should result in a rebuild.

PR-URL: https://github.com/nodejs/node/pull/6652


Reviewed-By: Anna Henningsen <anna@addaleax.net>