Nachos (Not Another Completely Heuristic Operating System) is an educational operating system.
You are reading the readme file for the Nachos version unr21a. This version was released in 2021 by teachers of the National University of Rosario.
In order to build userland programs, a cross compiler is needed. Some
GNU/Linux distributions already provide one as an installable package.
In Ubuntu, it's named gcc-mipsel-linux-gnu.