AIX testing strategy

At the moment we are exclusively testing AIX PRs against AIX 7.2.
AIX 7.3 has been available since December 2021 and we are not performing any testing on it.
This issue is to decide what approach we should use - how many machines we require etc.

I have confirmed that we can build node.js on AIX 7.3 and testing verification will be performed under

Assuming this works suitably well, we should decide on how many machines we need of AIX 7.2 and 7.3 in order to keep the CI happy.

Performance testing of the builds with different concurrency (make -j values) has been performed in https://github.com/nodejs/build/issues/4226

At present we have four AIX 7.2 test machines - two at OSUOSL and two at IBM. There is another 7.2 system at OSUOSL waiting to be reinstalled and I would propose that we get it reinstalled with AIX 7.3.

We currently have two IBM-hosted machines running AIX 7.3. An older one currently with 8GiB of RAM that is already in the CI at test-ibm-aix73-ppc64_be-1 and the new one that I have been experimenting with recently at test-ibm-aix73-ppc64_be-3

The other decision to make is whether we need a full set of AIX 7.3 machines to test on or whether we just allow each PR test to run on any AIX machine and let it randomly choose between 7.2 and 7.3. This approach would mean we likely wouldn't need as many systems in the set.

Start by reviewing the AIX 7.2 and 7.3 CI machines listed in the issue, including test-ibm-aix73-ppc64_be-1 and test-ibm-aix73-ppc64_be-3, and read build issue 4226 for concurrency results. Determine the required machine coverage and testing allocation; done means an agreed AIX testing strategy for CI.