This is just a modified version of my ASM-Playdate example. I removed the fast_sqrt.s variants, and added fast_sqrt.f90 instead. Note that the signature in fast_sqrt.h changed because Fortran uses double by default. The source code for the ASM project can be found in this post.
EDIT: Having said that, it should be easy enough to call the double fast_sqrt(double) function from any project that uses the C API. It is likely easier than fuzing around with my project, seeing as there is not a working Makefile for any of the Fortran stuff.