add ppc64/pwr8 as target
includes handling DIR_PWR8 where appropriate
The P7Model Itinerary is currently tied in for use under the P8Model, and will be updated later.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211779 91177308-0d34-0410-b5e6-96231b3b80d8