Added support for process waiting for another process to attach

This PR adds a new rvm flag (`WaitForAttach`) that let's the process sleep until another process attaches to it. The other process must have memory write access to this process to set the global attachFlag to a value != 0.

A debugger waiting for a process to launch may not be able to interrupt the process on its first instruction. We thus need the above functionality to synch the debugger and the inferior.

Happy to change the names of the flag and the global. I couldn't come up with anything better.