[llvm] Do not use R12 for indirect tail calls with PACBTI (PR #82661)

[via llvm-commits]

ostannard wrote:

What do you mean by "doesn't work"? I can see that you've added tests which use inline assembly to force the function pointer into `r12`, and they appear to generate correct code. Does clang generate different IR to `pacbti-indirect-tail-calls.ll`, and if so what is wrong with it?

https://github.com/llvm/llvm-project/pull/82661