debian/x86_64_build-base was missing the package that provides
LLVMConfig.cmake, breaking cmake builds that need LLVM, like the
SPIRV-LLVM-Translator build.
The cross-builds were missing arch-test to allow dpkg to figure out
which arch it's running on.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31940>