A check for 8-byte atomics is needed, as 32-bit platforms may require `libatomic` linking

[barracuda156]

Undefined symbols:
  "___atomic_fetch_add_8", referenced from:
      __ZN2ix15ConnectionState9computeIdEv in IXConnectionState.cpp.o
      __ZN2ix18WebSocketTransport13sendHeartBeatENS_15SendMessageKindE in IXWebSocketTransport.cpp.o
ld: symbol(s) not found

[bsergean]

We could make it a 32 bits atomic if that helps.

[barracuda156]

@bsergean Yes, if 8-byte atomics isn’t used, we won’t need libatomic linking.