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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> > [access-softek/musl@`dkovalev`/pauth-code-drop](https://github.com/access-softek/musl/tree/dkovalev/pauth-code-drop?rgh-link-date=2024-03-22T20%3A04%3A23Z) may need an update
> 
> @MaskRay This musl implementation is just a proof-of-concept to ensure that AUTH relocs work properly and to have ability to actually execute pauth-enabled code. In terms of this PoC, reading (platform,version) values from GNU property and faulting on their mismatch in different DSOs is considered out of scope - it does not give that much value in terms of testing the toolchain itself. So, we only have crucial features implemented - particularly, handling AUTH relocs.

Thanks.

The subtle `.relr.auth.dyn` support makes we wonder how much benefit we should expect from introducing a new relocation format.
I have filed https://github.com/ARM-software/abi-aa/issues/252 for quantifying the benefits of `DT_AARCH64_AUTH_RELR`.

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