Optimize tight CAS loops with x86 PAUSE / ARM YIELD instructions.

Thiago Macieira recently merged the addition of an _mm_pause() in a tight compare-and-swap loop in QFutureInterface: https://codereview.qt-project.org/c/qt/qtbase/+/407431

He links to various sources that detail why that is a good idea.

But we have many such loops in Qt, and adding that Q_PROCESSOR #ifdef'ery is making the code ugly. And besides, ARM has the YIELD instruction, which is serving the same purpose.

So, we need to have a function, say qYieldCpu(), to encapsulate all this, and then roll this out to other CAS loops, too.