The return address stack (RAS) is restructured to be a separate SimObject.
This enables disabling the RAS and better separation of the functionality.
Furthermore, easier statistics and debugging.
Change-Id: I8aacf7d4c8e308165d0e7e15bc5a5d0df77f8192
Signed-off-by: David Schall <david.schall@ed.ac.uk>