This adds a `.host-arch` symlink to build output; this doesn't get committed (for hopefully obvious reasons), but allows for things like `CMD` and `update.sh` to have a known-effective target for testing the output further in some way.
This was a mapping we already had thanks to `ARCH_TEST`, so I've also reordered our builds such that they're grouped by "host" architecture and sorted by "preference"/compatibility (with the goal that we get a more "correct" `.host-arch` symlink).
The intent of the previous implementation was to avoid libc, but it turns out that just invoking a syscall without libc is complicated (see https://github.com/docker-library/hello-world/pull/62#issuecomment-568573535 for details).
On the other hand, my personal machine can cross-compile all of musl in ~30s per architecture, which is pretty reasonable, and the resulting binary sizes are only around ~10k each, and I was able to do so successfully for every architecture we currently support.
Tianon Gravi