loader: Add prototype for gfx_interp_md

This function will be used to draw in the graphics bindings when the
loader is compiled with graphics (gfx) support. Provide definitions
for lua and the simple interpreter. 4th support is forthcoming.

Sponsored by: Netflix
Reviewed by: kevans, jhb
Differential Revision: https://reviews.freebsd.org/D43903

(cherry picked from commit 60e199d9fde1c3c60a96b969bf6982278cbc1e88)