Make `make cross-target' work in three stages.

Make `make cross-target' work in three stages.

1. moc->com in runtime, so the next stages can use a compiled runtime
2. moc->com in the whole tree
3. fni->inf in the whole tree

XXX Maybe the latter stages can be combined; for now this change just
splits the first two apart.  Even better, maybe we can nix this step
altogether and teach the cross-fasdumper to do this part itself.
Wouldn't be hard to teach the cross-fasdumper to do the relocation
that currently requires sketchy with-absolutely-no-interrupts crap.
For compile-scode, could maybe just fasdump to a bytevector in memory
and invent a primitive to fasload from that.