[clang] [llvm] [PAC][Driver] Implement `-mbranch-protection=pauthabi` option (PR #97237)

[Peter Smith via cfe-commits]

https://github.com/smithp35 commented:

As I understand it we are implicitly defining the default signing schema for ELF platforms with `-mbranch-protection=pauthabi` . 

Is there any thought on how we want to manage signing schemas going forward? For example I can imagine looking an environment from the triple to select a signing schema for a particular platform. I could also see a potential for a separate command line option to choose from documented named signing schemas.

I don't think these need to implemented now, would be good to make sure that there are enough comments stating the ABI implications of making changes.

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