Research whether libc++ and MS STL can map std::atomic to non-locked versions for bootstrap

We use more and more std::atomic instead of QAtomic in our code, and QTBUG-103847 proposes to ditch the bootstrap backend of QAtomic in favour of leaving only the C++11 version, so we should research whether there's a way to restore the QAtomic bootstrap optimisation of using int for QAtomicInt.

At least libstdc++ already optimizes std::atomic<> to lock-less versions if the application is single-threaded. Find out whether there's way to make the MS and the Clang libraries map std::atomic to non-locked versions, too, and, if so, enable that for the build of the bootstrapped tools.