doc: update AIX to GCC8 for v16.x

PR-URL: https://github.com/nodejs/node/pull/37677
Refs: https://github.com/nodejs/build/issues/2445


Reviewed-By: Stewart X Addison <sxa@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>