[lld] [lld][AArch64][ELF][PAC] Support `.relr.auth.dyn` section (PR #87635)

[via llvm-commits]

zeroomega wrote:

> @zeroomega Thanks for a detailed report!
> 
> > You can see index 16, `.rel.dyn` section is empty
> 
> Having rel(a) or relr section empty in some rare cases is expected behavior with this patch applied, we have tests for that (unfortunately, it's pretty hard to avoid that at current lld state). But, if such files cause issues when passed later as input for lld, we need to investigate that. If it does not contradict with specs, lld should be able to properly process input files with empty relocation sections, and it looks like that it isn't able to do that now.

I think the main issue is an empty REL(A) section.  This patch causes lld to generate a REL section with `sh_info` set to 0. IIRC this shouldn't happen for relocatable linking. And that is what lld complains in the later stages.

https://github.com/llvm/llvm-project/pull/87635