Display -mcpu/-mattr help even with no source files given.

Unfortunately, we can't easily add a test for this in our
current test runner setup (i.e. dmd-testsuite).

GitHub: Fixes #505.
Remove Walter-ism that doesn't even work in release mode.
Merge pull request #508 from klickverbot/mcpu-mattr-help

Display -mcpu/-mattr help even with no source files given.