Add an extra line before the "Additional Help" in rustc's --help output
auto merge of #9271 : brson/rust/extra-help, r=catamorphism

Look like this now

```
    -Z FLAG             Set internal debugging options
    -v --version        Print version info and exit

Additional help:
    -W help             Print 'lint' options and default settings
    -Z help             Print internal options for debugging rustc

```