fix NEG instruction to avoid UB and set AF properly

[pinterior]

SF is 0 (should be 1) on VS2019+clang build when negating 0x80000000.

Changes

• use subtract routine to avoid signed integer overflow (undefined behavior)
  • the routing also set AF properly

Check Program

https://gist.github.com/pinterior/443eeeda34b92e4206726a90b9bc4724

Closes #44 (not a clang bug)

[captainys]

Thanks, I take it.