Remove AIX support

[pcanal]

AIX has not been tested in a very long while (like only v5 and older) and is not yet supported enough by LLVM, see https://llvm.org/docs/JITLink.html#roadmap:

Major outstanding projects include:

    Improve XCOFF support.

    The XCOFF/ppc64 backend exists but does not yet implement the relocation handling needed for general JIT use. Completing this would enable JITLink on AIX.

In addition, all the work-around are likely obsolete and/or have better alternative in AIX 7.3+ and thus should be redone if (when?) there is demand for a port to AIX.

[github-actions]

Test Results

    23 files      23 suites   3d 14h 9m 13s ⏱️
 3 857 tests  3 856 ✅ 0 💤 1 ❌
78 593 runs  78 592 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 03e2663.

[guitargeek]

Thanks a lot for the initiative! I think this only needs some fixes in roottest/scripts/Event.mk, then it's ready to merge.

[guitargeek]

The nested ifeq-else-endif triplets are broken now. You also have to remove the trailing else after the line you just removed, and the two endifs corresponding to the ifeqs you removed

[dpiparo]

lgtm when @guitargeek's comments are addressed.
For when is the the cleanup foreseen for preprocessor guards for discontinued OSes, e.g. macOS versions :-P?