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

[Sean Perry via cfe-commits]

perry-ca wrote:

@MaskRay no worries.  This brittle test issue was a reason I trying to keep the default for zLinux unchanged by this.

I have posted a new commit that eliminates the cmake variable for z/OS.

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