[llvm][clang] Correct example in -mcpu-help text

This used to say:
For example, clang --target=aarch64-unknown-linux-gui -mcpu=cortex-a35

Which works but I think it was meant to be `-gnu` not `-gui`. From my
AArch64 Linux build:
```
$ ./bin/clang --version
clang version 17.0.0 <...>
Target: aarch64-unknown-linux-gnu
```

Originally added in af857b9.

Author: DavidSpickett

llvm/lib/MC/MCSubtargetInfo.cpp

@@ -142,7 +142,7 @@ static void cpuHelp(ArrayRef<SubtargetSubTypeKV> CPUTable) {
   errs() << '\n';
 
   errs() << "Use -mcpu or -mtune to specify the target's processor.\n"
-            "For example, clang --target=aarch64-unknown-linux-gui "
+            "For example, clang --target=aarch64-unknown-linux-gnu "
             "-mcpu=cortex-a35\n";
 
   PrintOnce = true;