PowerPC bugfixes:
- must clear carry bit when doing addic with a zero immediate value
- fix missing RETURN in micro-operation that would lead to random failures
  and crashes
- add USE_PRECISE_EMULATION compilation-time option to choose between
  getting exact floating point results and fast but less accurate computation.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@2526 c046a42c-6fe2-441c-8c8c-71466251a162
