Keep the mark chunk pop locks exclusive

[Tutez64]

#1389 turned the two pop spin locks in GlobalChunks into:

int expected{ 0 };
while(false == processListPopLock.compare_exchange_strong(expected, 1))
{
   // Spin
}

compare_exchange_strong stores the current value in expected when it fails. After the first failed attempt expected is 1, so the next attempt compares the lock against 1 and succeeds while another thread still holds it. The old _hx_atomic_compare_exchange(&lock, 0, 1) compared against 0 every time.

With the lock no longer exclusive, two marking threads can pop processList (or freeList) at once, which is the ABA case the lock exists to prevent: a thread's compare_exchange can move the head onto a chunk another thread has just taken, so a chunk is processed twice or lost. A lost chunk leaves objects marked but never traversed, and whatever only they reference is freed.

We hit this in a game built with HXCPP_GC_GENERATIONAL after updating hxcpp past #1389: intermittent crashes on native access to freed memory (a MovieClip's __children buffer). With HXCPP_GC_CHECK_POINTER the first bad access was Old object access on a field of an object whose own access had just passed the check, i.e. a marked object whose child had not been marked. The crash does not occur with marking on a single thread, nor with hxcpp from before #1389, nor with this change; I could not reproduce it in a standalone stress test, the interleaving it needs being rare outside the game.

The fix resets expected to 0 on every spin, in both loops. mZeroLock and gPauseForCollect use a single attempt and are unaffected.