The fix to the atomicArc looks to use `-1` as the check value from the
`SharedPtr` solution. However, it should be `-rcIncrement` since the
refcount is bit shifted in ARC/ORC.
I discovered this playing around doing atomic updates of refcounts in a
user library.
Related to https://github.com/nim-lang/Nim/issues/22711
@ringabout I believe you ported the sharedptr fix?