Unsoundness of AtomicCell<*64> arithmetics on 32-bit targets that support Atomic*64

Impact Affected versions of this crate incorrectly assumed that the alignment of i,u64 was always the same as AtomicI,U64. However, the alignment of i,u64 on a 32-bit target can be smaller than AtomicI,U64. This can cause the following problems: - Unaligned memory accesses - Data race Crates usin...