[llvm] [PowerPC][AIX] Enable aix-small-local-dynamic-tls target attribute (PR #86641)

[Felix via llvm-commits]

orcguru wrote:

> Some initial comments for now. Will a subsequent front end patch also come?

Hi Amy, subsequently I will follow your commits (b1922e55ab3b and then 2a50921553798). The next one will enable the functionality through front end by adding "-maix-small-local-dynamic-tls" clang option. After that, I will refactor some existing logic to handle non-zero offsets for TLS local-dynamic.

Let me know if you have other thoughts. Thank you!

https://github.com/llvm/llvm-project/pull/86641