PowerPC flags update/use fixes:
- fix confusion between overflow/summary overflow, as reported by S Bansal.
- reset carry in addic. optimized case (as it was already done in addic).


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3179 c046a42c-6fe2-441c-8c8c-71466251a162
