When building universe ensure the required worlds are finished before
starting the kernels. Before this the kernels would be built as part of the
last architecture universe target. There can cause problems when this world
finishes before the other worlds as the host compiler may be picked up
rather than the target compiler.

The solution is to add a target to build the universe kernels that depends
on all the world targets finishing. As we may not be building a world only
depend on it when MAKE_JUST_KERNELS is undefined.