[clang] [PAC][clang] Handle pauthtest environment and ABI in Linux-specific code (PR #113151)

[Anton Korobeynikov via cfe-commits]

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

`pauthtest` currently is a Linux-specific ABI and we do not want to expose it further on. Even more, any signing scheme option currently exposed effectively defines a new *binary incompatible* ABI. 

`pauthtest` is intended for quick testing during adoption and for other platforms should they want to deploy pauth to use as a template. We do not want to make it something end users would depend on.

So, let's more the code down to particular platform and remove platform-specific checks from generic code.

https://github.com/llvm/llvm-project/pull/113151