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

[Dimitri John Ledkov via cfe-commits]

xnox wrote:

Requiring a config file when previously was not needed is a regression in behaviour for distributions.

I guess Ubuntu would have to continue to patch this back in.

Built-in defaults exist for a reason and yes at times need to be higher.

This change risks performance regressions for RHEL SUSE and Ubuntu. Please call out this change in release notes.

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