Sidedness not verified with Mixins configs

[gabizou]

In reference to issue: SpongePowered/SpongeForge#2310

The change referenced in this commit provides an issue where one could determine a mixin is necessary, regardless of platform.

I think this can be a good simple check based on the target of the mixin against the currently existing sided checks.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gabizou]

it's not stale....