Fix 16-bit mode instruction pointer wrap-around

Not all x86 CPUs are even able to run in 32-bit mode, which creates portability issues.

Specifically, the problem I'm facing which prompted me to open this issue is running code that relies on the fact that on 16-bit mode when `ip` is 0xFFFF it'll wrap back to 0, `v86` breaks this functionality as it uses `eip` causing it to continue to `0x10000`.