[lld] [lld] Support thumb PLTs (PR #86223)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> For Cortex-m PLTs presumably for shared-libraries? Aren't a great fit due to the lack of an MMU. Although much more work to implement the fdpic ABI [mickael-guene/fdpic_doc@`master`/abi.txt](https://github.com/mickael-guene/fdpic_doc/blob/master/abi.txt?rgh-link-date=2024-03-22T11%3A30%3A36Z) may be a better fit.

I am curious about the use case as well. I have a fdpic WIP implementation [for my FDPIC study](https://maskray.me/blog/2024-02-20-mmu-less-systems-and-fdpic) that hard codes the FDPIC register r9, which does not yield the best performance. A better implementation is to make r9 like a hidden argument that is call-clobbered.

(BTW: gcc's arm fdpic does not handle r9 very well. gcc's sh4 port handles r12 in a better way.)

https://github.com/llvm/llvm-project/pull/86223