Boost's b2 clang toolset compiles with --target=arm64-pc-linux on aarch64.
Clang cannot map that triple to Arch Linux ARM's aarch64-unknown-linux-gnu
GCC install, so every Boost configure check and compile lost libstdc++
headers and crtbeginS.o. Ship an arm64-pc-linux.cfg next to clang that sets
--gcc-triple to the host GCC.
archlinux/archlinux only publishes linux/amd64, so the native aarch64 Arch
lane failed with 'exec format error' before provisioning. Both Arch
Dockerfiles now take ARCH_BASE_IMAGE; the aarch64 matrix entry pins
ghcr.io/menci/archlinuxarm by digest and x86_64 keeps the official image.
package-validate.sh forwards ARCH_BASE_IMAGE to the systemd substrate.
