Upgrading "dead" Weak< UnsafeCell< MaybeUninit<T>>> to Rc<T>

[programmerjake]

@steffahn

as the reference counters can only be updated in a Relaxed manner with the public API of Arc, which is likely insufficient for sound initialization.

all you need to effectively convert the Relaxed atomic to a Release atomic is to put a Release thread fence after all writes and before the Relaxed atomic.