PowerPC MMU and exception fixes:
* PowerPC 601 (and probably POWER/POWER2) uses a different BAT format than
  later PowerPC implementation.
* Bugfix in BATs check: must not stop after 4 BATs when more are provided.
* Enable POWER 'rac' instruction.
* Fix exception prefix for all supported PowerPC implementations.
* Fix exceptions, MMU model and bus model for PowerPC 601 & 620.
* Enable PowerPC 620 as it could mostly boot a PreP target.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3518 c046a42c-6fe2-441c-8c8c-71466251a162
