[clang] [llvm] [PAC][Driver] Support `pauthtest` ABI for AArch64 Linux triples (PR #97237)

[Peter Smith via cfe-commits]

https://github.com/smithp35 approved this pull request.

After discussion in the PAuthABI call. We agreed that it would be best to have an exemplar of how a signing schema for a platform should be encoded rather than always using the individual options.

Anton mentioned that we can document that pauthtest is not a stable signing schema to be used in production, and if there ended up being a stable upstream linux signing schema -mabi=pauth (probably) then pauthtest could be removed.

https://github.com/llvm/llvm-project/pull/97237