gh-132868: use _Alignof() operator in the struct module

[skirpichev]

• Issue: Use _Alignof to query alignments in the struct module #132868

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @picnixz for commit 10eb674 🤖

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https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F132872%2Fmerge

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[picnixz]

The refleak failure is likely unrelated but I don't know whether it's a known issue or not. Anyway, for this one, I'll assume that C11 is supported.