Update: I got SNES working. (Note: The pre-built core linked here did not work: RetroArch Megathread - #263 by infernalmaster)
git clone https://github.com/libretro/snes9x2010
cd snes9x2010
make
chmod -x snes9x2010_libretro.so
sudo cp snes9x2010_libretro.so /usr/lib/aarch64-linux-gnu/libretro/
Sound is pretty choppy (I’ve already increased the Audio Latency to 128 and set the Audio Resampler Quality to Lowest, or Don’t Care – no improvement). But this doesn’t seem to be specific to SNES.
I don’t seem to be able to attach a file here, or I would have attached the working binary.