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.
Add counter to keep track of the number of timer interrupts generated by
the local apic for each virtual cpu.
Fix xdev. Clang and libc were not building correctly on older worlds, for
example, on 9.1.
 * To fix clang add an _xb-bootstrap-tools target that mirrors the existing
   bootstrap-tools target in the full world.
 * For libc have the compiler use the newly installed includes, and, while
   here, tell the compiler about the xdev library path as some other
   libraries will link against the installed libraries.
Fix xdev-install when installing to a location other than /