mac:  Add _ExitToShell
Makefile:  Remove xv68k-{ExitToShell,beep} targets

We're removing Mac Toolbox code from xv68k.  The mac loader supports the
_ExitToShell and _SysBeep traps, but until we figure out how that should
be packaged, it's easiest just to remove the call sites.
xv68k:  Don't install _ExitToShell, _SysBeep traps
v68k-callbacks:  Remove ExitToShell and SysBeep