build: remove AIX/ppc (32bit) dead code
- also refactor OS400 detection
Refs: https://github.com/nodejs/node/pull/25447/files/36839defcfaf7c46435e16fb1f0da006f3ebe8ac#r247378894
/CC @nodejs/build-files @nodejs/platform-aix
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines