[PowerPC] Support time base instructions

This adds support for the old-style time base instructions;
while new programs are supposed to use mfspr, the mftb instructions
are still supported and in use by existing assembler files.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185829 91177308-0d34-0410-b5e6-96231b3b80d8