Cite a paper currently under review
A common way to handle such a previous submission that is still under review is to create a technical report out of the previous submission, publish this report on your home page or on arxiv.org, and cite this technical report in your new paper.

In this way, reviewers can take into account the previous results without needing to have this paper published formally.

In case your previous paper gets accepted while your new paper is still under revision, you could update your new paper in a revised version.