[PAC] Make libunwind pauth-friendly

Downstream commit: https://github.com/ahmedbougacha/llvm-project/commit/2392341fa7d5d5bf40398b63cc4fd65aeaf055cf
ELF-related fixes: https://github.com/access-softek/llvm-project/commit/63d119a482e3d09c8e1b97c1532712c0ab9cc34c