I use https://github.com/pure-data/pd-lib-builder for the templates for my makefiles, as its got windows/mac/arm support, so I build on a mac, then once I’m happy I transfer them over to the Organelle and compile there.
externals are generally pretty small, so its generally not really worth the effort to setup a cross compiling environment.
Organelle already has build tools installed, but its useful to have a wifi network setup, so you can do things like git pulls and be able to ssh into it.
if you are converting existing externals with makefiles for arm you want to add
-march=armv7-a -mfpu=vfpv3 -mfloat-abi=hard
(and obviously move any x86 specific flags)
its all pretty straightforward really, not rocket science