If I package an Android app with Google ARC Welder and distribute it through the Chrome Web Store, will it be available for Windows, Mac, Linux, or only for Chrome OS? If so, are there any news if that will change in future?
It's ChromeOS only, as clearly stated at https://developer.chrome.com/apps/getstarted_arc :
The App Runtime for Chrome (Beta), or ARC, lets you run your favorite Android apps on Chrome OS
There is no information about support for other platforms other than for development and initial testing at this time.