Comment 7 for bug 1665447

The mainline builds are all cross-built rather than native
built. Binaries like fixdep etc are used by the build process, and since its a
build on amd64 those binaries are complied for amd64. It is very resource
intensive to build these unsupported kernels native for each architecture.

Also note that these kernels are not intended for production systems, and are
made available for sneak peek/testing purposes only. These are unmodified
upstream kernels built using the ubuntu config files. Or in other words.. these
packages are "install at your own risk" and not supported in any way.