[clang] [SystemZ/[z/OS] Guard source-epoch related testcase to 64-bit clang targets (PR #210341)

[Abhina Sree via cfe-commits]

================
@@ -1,3 +1,5 @@
+//REQUIRES: clang-target-64-bits
----------------
abhina-sree wrote:

```suggestion
// REQUIRES: clang-target-64-bits
```

https://github.com/llvm/llvm-project/pull/210341