Interp atomicrmw - #5993

sync/atomic functions are compiled to ordinary bodies built on atomicrmw
and cmpxchg, so interpreting those instructions covers Add, And, Or, Swap
and CompareAndSwap for every type without matching on function names.

Before other execution contexts exist an atomic operation on memory the
interpreter owns is just a load and a store. Volatile operations and
memory with external loads or stores are still left for runtime, as with
the existing atomic load and store handling.