The avr-libc at least comes with a malloc (rather, heap) implementation. It's mostly a trap for new embedded programmers who haven't yet internalized that you shouldn't touch dynamic memory allocation with a stick on these kind of processors.

(Embedded programming is really quite nice to teach clean C, you learn to appreciate what the cost of library functionality is, that you can't just start using floats, that on some architectures updating a 32 bit integer is a non-atomic operation spanning 5 or more clock cycles..)