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

[Fangrui Song via cfe-commits]

================
@@ -11,6 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "clang/Config/config.h"
----------------
MaskRay wrote:

If SystemZ.h uses CLANG_SYSTEMZ_DEFAULT_ARCH, that file should have `#include "clang/Config/config.h"` as IWYU

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