[clang] [SystemZ][z/OS] Mark the -msoft-float option as unsupported on z/OS (PR #204209)

[Zibi Sarbinowski via cfe-commits]

================
@@ -0,0 +1,3 @@
+// RUN: not %clang -msoft-float --target=s390x-unknown-zos 2>&1 %s | FileCheck %s
+
+// CHECK: error: unsupported option '-msoft-float' for target 's390x-unknown-zos'
----------------
zibi2 wrote:

```suggestion
// CHECK: error: unsupported option '-msoft-float' for target 's390x-none-zos'
```

https://github.com/llvm/llvm-project/pull/204209