Move bits dealing with Xen into a xen/ subdirectory.
Add libs as a submodule instead of manual pulling.

I called it "nblibs" instead of "xen-nblibc" (per the repo name) since
the repo contains more than just libc and it's not really Xen-specific.

... and yes, we still want to get rid of that repo and be able to
use unmodified upstream sources.
make clean for all subdirectories