[clang] [z/OS] Set the default arch for z/OS to be arch10 (PR #89854)

[Sean Perry via cfe-commits]

perry-ca wrote:

@MaskRay  Got it.  

The problem with that solution is that if you use --target you won't get the correct arch.  This would be a problem for any cross compilation.  For example, say you cross compile from zLinux (which wouldn't have the config file), the arch would be arch8.  And if you cross compiled from z/OS (with the config file) to zLinux then the default arch would be arch10.  Both of these would be incorrect.

We also use the config files for installation specific information.  It is common for users to have their own config files.  If we require that the option be in the config file then we would be creating a very error prone situation.

We need to be able to change the arch default based on the target triple.

https://github.com/llvm/llvm-project/pull/89854