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>

doc/onboarding-extras.md

@@ -76,10 +76,10 @@ Please use these when possible / appropriate
 ### Other Labels
 
 * Operating system labels
-  * `os x`, `windows`, `solaris`
+  * `os x`, `windows`, `solaris`, `aix`
   * No linux, linux is the implied default
 * Architecture labels
-  * `arm`, `mips`
+  * `arm`, `mips`, `s390`, `ppc`
   * No x86{_64}, since that is the implied default
 * `lts-agenda`, `lts-watch-v*`
   * tag things that should be discussed to go into LTS or should go into a specific LTS branch