Yeah the cross-compilation thing is definitely a rough spot. I have one project that's able to work around it via extensive hacks with macros, but at some point I'll need to do it "the right way."
Honestly if the docs had a canonical example of e.g. using unix_cc_toolchain_config (example: [0]) + Bootlin to compile for aarch64, it'd probably go a long way to making things understandable. Because say what you will about the old CROSSTOOL approach, at least there was a nice tutorial for it.
[0] https://github.com/grailbio/bazel-toolchain/blob/f14a8a5de8f...