Re-enable debug breaks

Int3 was defined as a no-op everywhere except for Mac, where it was calling Debugger(), which is deprecated.
This re-enables it for MSVC and GNUC on x86 or amd64 (the only platforms that have the literal int 3 cpu instruction) 

There are other methods of breaking which could be used on other platforms if desired. See https://github.com/scottt/debugbreak for examples and caveats.

You could also just delete the Int3 from inside of multi_do_frame to stop it from hitting the breakpoint all the time. I chose to wrap it in an if because that's the established idiom everywhere else multi_do_frame was called.