It seems that xcomp can't manage to reliably compile core+ units.
Let's limit the role of xcomp to z80c+icore.
I'm getting a little desperate here. I'll revisit the link.fs approach
with a new idea I had...
... instead of building boot.fs, and then z80c.fs/icore.fs in separate
passes. This strengten xcomp by making it support 2 completely separate
dicts (that is, the X dict is *not* connected to the system dict. The chain
now stops at EXIT def in boot.fs).
