Fix lock-ordering during init

[ERobsham]

Reorders how locking is handled in objc_send_initialize() to prevent a deadlock. Previously, contention on the low level spinlocks could cause a very intermittent deadlock:

• Thread A : objc_send_initialize() holds the runtime lock, then tries to acquire the object lock on the metaclass, which needs to initialize the mutex for the new metaclass inside referenceListForObject(), so it tries to lock the lock_for_pointer() / lock_spinlock()

• Thread B : referenceListForObject() holds a spinlock for an unrelated object while running initHiddenClassForObject() -> allocateHiddenClass(), which tries to acquire the runtime lock

  If the metaclass object pointer in Thread A hashes to the same spinlock as the object in thread B, the runtime lock ends up deadlocked forever.

  Potential fix for this issue

[davidchisnall]

Thanks, I think we need to keep the current lock acquisition to avoid UB but then drop it before acquiring the second one and reacquire it lower down.

[ERobsham]

Thanks for the speedy response! Im not 100% sure what you mean by "to avoid UB"? So let me know if this update was the correct change you're looking for.

[davidchisnall]

Looks great, thanks!