Hi guix.

when running following command, the error message is not expected.

```
citreu@asus-laptop:~$ guix build hello --target=foobar
guix build: error: 'foobar' is not a supported cross-compilation target
hint: Try `--list-targets' to view available targets.~%

citreu@asus-laptop:~$ guix build hello --system=foobar
guix build: error: 'foobar' is not a supported system
hint: Try `--list-systems' to view available system types.~%
```

Unexpected ~% at the end of message.

In guix/scripts/build.scm, the code use display-hint to show this hint
but it doesn't format the "~%" to newline.

This patch adds these missing formats