Support building on an arm64 host too

[tianon]

cc @LaurentGoderre

(the rest of the image build is appropriately using the full compiler prefixes and arch-test to verify executability, even on amd64)

[tianon]

Doh, this will fail without emulation too:

hello-world/update.sh

Lines 23 to 29 in 6ad8af5

	for h in amd64/*/hello; do
	d="$(dirname "$h")"
	b="$(basename "$d")"
	"$h" > /dev/null
	docker build -t hello-world:"test-$b" "$d"
	docker run --rm hello-world:"test-$b"
	done

I wish we had an easier way to invoke https://github.com/docker-library/bashbrew/blob/86212cac9100bf6163bc91a0560e7cbc6b1da995/scripts/bashbrew-host-arch.sh, which I guess would have to happen inside a container too since that's the real "arch" we're testing here (and may even be a remote system) 😭

[LaurentGoderre]

I think this might be good enough for a first step?

[tianon]

I think I can do something ~simple to resolve it 😅

[tianon]

Oh, and we don't need https://github.com/docker-library/bashbrew/blob/86212cac9100bf6163bc91a0560e7cbc6b1da995/scripts/bashbrew-host-arch.sh because we already have the mappings, we just have to use/record them somewhere 😅 😇

[tianon]

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).