Previously the remaining use of the C implementation on 32-bit platforms
resulted in a subtle bug, #24261. This was due to the C object (which
used the RTS's `atomic_inc64` macro) being compiled without `-threaded`
yet later being used in a threaded compiler.

Side-step this issue by using the pure Haskell `genSym` implementation on
all platforms. This required implementing `fetchAddWord64Addr#` in terms
of CAS on 64-bit platforms.