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

[Fangrui Song via cfe-commits]

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@@ -4141,6 +4141,20 @@
 
 // Begin SystemZ/GCC/Linux tests ----------------
 
+// RUN: %clang -E -dM %s -o - 2>&1 \
+// RUN:     -target s390x-ibm-zos \
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MaskRay wrote:

`--target=` for new driver tests.

However, this does not test clangDriver and therefore `%clang_cc1` is preferred.

(You can ignore previous lines that obey the convention. I can fix them later.)

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