Support int3 on gcc

This branch does two things, it puts [DEBUG] in the version string if it's a debug build, and it makes the Int3() function actually trigger a software breakpoint interrupt. If you're running in a debugger on a debug build and you hit an Int3() call, you'll actually break where it happened.  If you're running a debug build and Int3() is called outside of the debugger the program halts. This requires inline assembly, which the Microsoft compiler doesn't support in 64bit mode, so for now this is _GNUC_ only.