Optimize PowerPC overflow flag computation in most useful cases.
Use the same routines to check overflow for addo, subfo and PowerPC 405
  multiply and add cases.
Fix carry reset in addme(o) and subfme(o) cases.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3574 c046a42c-6fe2-441c-8c8c-71466251a162
