PPC: Refactoring to support subtarget feature changing

This change mirrors the changes that were made to the X86 and ARM targets to
support subtarget feature changing. As indicated in r182899, the mechanism is
still undergoing revision, and so as with the X86 and ARM targets, there is no
test case yet (there is no effective functionality change).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186357 91177308-0d34-0410-b5e6-96231b3b80d8