I was able to get Nim working in Termux after some trial and error. [Stisa's article](http://stisa.space/articles/compile-nim-on-android/) helped with some of it, but I think I had to implant some additional code. I also had to [patch getTempDir](https://github.com/nim-lang/Nim/issues/5260) to get nimble/etc working on an unrooted device.
I wouldn't recommend doing Nim compiling directly on a mobile device though. Nim is a particularly smart compiler that does a lot of work to go from a superb syntax to a great fast executable, which is awesome on all fronts but one: it means noticeable compile times on low-power devices. Developers should use real computers with fast multicore CPUs, or at least a fast on-demand CaaS (compiler as a service) in the cloud. One can make a script to cross-compile on a fast device and deploy to the mobile device for testing.
