[llvm] [PowerPC] Add bctar instruction (ISA 2.07) (PR #187322)

[Jacob Lifshay via llvm-commits]

programmerjake wrote:

> Is this needed for something specific or just ISA completeness?

I needed it when trying to assemble bctar using llvm: https://github.com/llvm/llvm-project/issues/176831#issuecomment-4101798392

https://github.com/llvm/llvm-project/pull/187322