doc: add s390 and ppc architecture labels

Adds `s390` and `ppc` as Arch labels, and `aix` as an OS label.
PR-URL: https://github.com/nodejs/node/pull/9009


Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>