AmigaOS: Add a stack cookie

Adding a static stack for users who either doesn't set the right amount (i.e. starting from shell) or using a too small stack inside the icon (defaults to 600.000 aswell, but you never know).
Right now ScummVM needs approximately 300.000 bytes. A double sized stack should be safe for now.
As ScummVM grows bigger and gets new engines, i will review the stack size.