Esoteric microcontrollers don't need an ABI. ABIs are for static and dynamic linking, and in the microcontroller world, there isn't a whole lot of linking going on. It's very common to statically compile everything into one single flash image, with libraries being in source form. E.g. this is the case in the Arduino ecosystem.
I don’t think there is anything wrong with the single compilation unit approach, but every embedded project I’ve been on on the last 10 years has used static linking, which doesn’t preclude ending up with a single flash image. 10 years is about the last time I worked on a non-arm based project, so that could be a factor (the pic compiler I was familiar with at that time did not have a linker, and I’m not sure if the avr compiler I used did)
They still have an internal ABI used when static linking; aligning structures, laying out data sections, function call behavior, and so forth. C's flexibility/ambivalence in this regard is why it's preferred on those architectures.
One could consider static linking in this scenario just an optimization for incremental builds with separate compilation of source files; it's not really used to, e.g. link a library that has no source. In that sense, it's just an implementation detail that's leaked from a build system that notionally recompiles from source but has unwisely exposed the units of its caching system.
Microcontroller programs are also pretty small; it's not difficult to load the whole program into memory on a workstation and compile the whole thing together. This is how Virgil (circa 2006) worked.
