Incorrect handling of QT_FATAL_CRITICALS/QT_FATAL_WARNINGS (non-UB data race)

The code that counts down the value of QT_FATAL_CRITICALS exhibits a data race. It's not UB, because the variables are atomics, but it nonetheless can lead to no qCritical()'s terminating the application, even though some N > 1 was requested:

        // it's fatal if the current value is exactly 1,
        // otherwise decrement if it's non-zero
        return fatalCriticals.loadRelaxed() && fatalCriticals.fetchAndAddRelaxed(-1) == 1;

This code must be thread-safe, obviously, but since the checks are separate atomic operations, they can interleave when two threads execute them concurrently (they might even be reordered, since they're relaxed atomic, IIRC). I'm hard-pressed finding an execution that can fail to detect zero, but the relaxed atomics are funny that way, so it's better to change this into a CAS loop, or count down past the 0 and use <= 1.