Refactor OS exit functions

[startLinux()](https://github.com/XboxDev/cromwell/blob/5d3d3f30ddd7aa3d3611c1bfb99e94954e59df52/boot/LoadLinux.c#L325) and [startReactOS()](https://github.com/XboxDev/cromwell/blob/5d3d3f30ddd7aa3d3611c1bfb99e94954e59df52/boot/LoadReactOS.c#L314) functions are different, but they have some similar code that needs to be factored out.