[llvm] [SystemZ][z/OS] Enable lit testing for z/OS (PR #107631)

[Zibi Sarbinowski via llvm-commits]

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@@ -843,6 +843,10 @@ size_t raw_fd_ostream::preferred_buffer_size() const {
   if (IsWindowsConsole)
     return 0;
   return raw_ostream::preferred_buffer_size();
+#elif defined(__MVS__)
+  // The buffer size on zOS is defined with macro BUFSIZ, which can be retrieved
----------------
zibi2 wrote:

```suggestion
  // The buffer size on z/OS is defined with macro BUFSIZ, which can be retrieved
```

https://github.com/llvm/llvm-project/pull/107631