Share kodata across multi-arch builds?

Talking to @jonjohnsonjr I realized that we might be able to save some cycles if we found a way to share the kodata layer across architectures in multi-arch builds. Right now we are deterministically producing the same layer N times, but it should be safe to share this if we can find a way to reasonably do so.

The issue names no files, tests, or entry points. Start by mapping how multi-arch builds generate and assemble the kodata layer, then determine whether identical layers can be shared safely across architectures; done means avoiding repeated layer production without changing build output.

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