So, I have assembly running on the devkit on both arm64 cores.
But I can't run C until I can compile C, and have a C runtime library.
Attempts to build newlib from scratch as a standalone library ended poorly (I got a makefile that did nothing, apparently other people have hit this and I never got a straight answer as to how one builds a naked cross-newlib, not as part of a compiler).
So now I'm trying to build a full aarch64-none-elf toolchain. Wish me luck.