Here are the references I know concerning this:

H. F. Baker, Examples of applications of Newton's polygon to the theory of singular points of algebraic functions, *Trans. Cambridge Phil. Soc.* **15** (1893), 403-450.

A. G. Khovanskii, Newton polyhedra and the genus of complete intersections, *Funct. Anal. i ego pril*. English translation: *Functional Anal. Appl.*, **12** (1978), 38-46.

V. I. Danilov and A. G. Khovanskii, Newton polyhedra and an algorithm for
computing Hodge-Deligne numbers, *Izv. Akad. Nauk SSSR Ser. Mat.* **50** (1986), 925-945; English translation: *Math. USSR-Izv.* **29** (1987), 279-298.

P. Beelen and R. Pellikaan, The Newton polygon of plane curves with many
rational points, *Designs, Codes and Cryptography* **21** (2000), 41-67.
(See Theorem 4.2.)

I think the statement should really be, given an irreducible curve in $\mathbf{G}_m^2$, a formula for the arithmetic genus of its closure in the 2-dimensional projective toric variety corresponding to the polygon. This way one should not need to impose genericity hypotheses or restrictions on the characteristic. The references above don't quite do all of this, however, so there is still room for a better reference or proof, I think.