Embed dpkg's factual cputable/ostable/tupletable/abitable data and
implement tuple/triplet/multiarch lookups, wildcard matching, arch
restriction evaluation and the full DEB_BUILD_*/DEB_HOST_*/DEB_TARGET_*
environment dump natively.
build/env.rs::arch_env now delegates to the native implementation
instead of shelling out to 'dpkg-architecture -f'.
Differential gate (build/mod.rs): arch_env(Some(a)) must equal real
'dpkg-architecture -f -a a' key-for-key for every architecture listed by
'dpkg-architecture -L', plus the native case. Data tables carry upstream
attribution comments; no dpkg code was transliterated.
