[3.13] gh-129259: Fix AIX build failures caused by incorrect struct alignment

[rsmkaminskas]

This PR fixes the AIX build failures mentioned in issue #129259.

Note In 3.14, the _Py_BackoffCounter bit field structure was replaced with a uint16_t

• Issue: 3.13 AIX build static asserts fails '(sizeof(_PyCallCache) / sizeof(_Py_CODEUNIT)) == 3': incorrect cache size #129259

[python-cla-bot]

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[rsmkaminskas]

Hi @encukou , could you please review this PR?

[encukou]

Oh, wait, I missed that this only fails on 3.13.
We generally don't backport fixes to maintenance branches, the same would apply to a 3.13-only fix. But we can make an exception; and I guess we should, for this hard-to-diagnose issue.

@ayappanec, as the AIX expert, does this look good?

[ayappanec]

I guess this issue happens with OpenXL compiler (IBM's clang compiler) which by default passes "-maltivec" that enforces strict alignment rules.
@encukou The changes looks good.