When built with Python 3.9 on IBM i, `process.platform` will return
`os400` instead of `aix`. In preparation for this, make `common.isAIX`
only return true for AIX and update the tests to add checks for
`common.isIBMi` where they were missing.
PR-URL: https://github.com/nodejs/node/pull/48056
Refs: https://github.com/nodejs/node/pull/46739
Refs: https://github.com/nodejs/build/pull/3358
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
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