[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 af857b93.