build: remove AIX/ppc (32bit) dead code

* also dedup OS400 detection

PR-URL: https://github.com/nodejs/node/pull/25523
Refs: https://github.com/nodejs/node/pull/25447/files/36839defcfaf7c46435e16fb1f0da006f3ebe8ac#r247378894


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>