NCG: Unreachable incorrect code in `outOfLineCmmOp`

Summary

Certain MachOps are implemented by calls to C libs.

However we have code in this function which:

• Implements emmision for a call which is never used (we will always generate inline code).
• If we were to use the call the symbol name would be incorrect.

As far as I can tell that's the case for these atomics

              MO_AtomicRMW _ _ -> fsLit "atomicrmw"
              MO_AtomicRead _  -> fsLit "atomicread"
              MO_AtomicWrite _ -> fsLit "atomicwrite"
              MO_Cmpxchg _     -> fsLit "cmpxchg"

There were all added in the same commit so that makes sense. They should use the labels in fashion like clzLabel does.