[lld] [lld][AArch64][ELF][PAC] Support AUTH relocations and AUTH ELF marking (PR #72714)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > To the best of my knowledge I'm not aware of any of these as this is the only serious implementation of the ELF spec.
> 
> I'm not aware of such implementations as well.
> 
> I suppose we can do the following:
> 
> * Update the spec and remove PT_NOTE from it. If someone needs it for some reason, they can ask for it explicitly.
> * Keep only GNU property support in lld.
> 
> @MaskRay @smithp35 Please let me know what do you think on this. I'll update the PR depending on your answers.

Thanks to @smithp35 for the background. I want to avoid `.note.AARCH64-PAUTH-ABI-tag` complexity.

Think: If the compiler emits both note sections, there will be small but extra overhead for each relocatable file. Some users may want an option to turn off `.note.AARCH64-PAUTH-ABI-tag`, then we end up with additional compiler driver complexity.

https://github.com/llvm/llvm-project/pull/72714