aarch64 and sparcv9 cross-build artifacts in the cache

Cross-compilation from the x86_64 build hosts now populates the cache with store paths for additional architectures. This means a Nix user targeting a non-primary arch can pull prebuilt artifacts instead of cross-compiling everything locally. Coverage is partial and grows as more of the package set cross-builds cleanly; platforms without a native builder still appear honestly as "no objects yet" rather than as coverage that does not exist.