One more round of rework for LIARC.

One more round of rework for LIARC.

Keep the host actions and target actions separated by a stamp file as
with other cross-compilation.

Name the targets:

- cross-host does the actions on the host (some day, all of them),
  and creates stamp_cross-host

- cross-target does the actions on the target, which all depend on
  the host actions only through stamp_cross-host

- all does cross-host and then cross-target.

With any luck, this should preserve the behaviour of non-cross
builds.

Still missing: Makefile.tools doesn't work _from_ a system with LIARC
yet to cross-compile, because we have no way to build a fresh runtime
or create bundles outside the standard build.

=> We should not depend on a full fresh runtime, only on the macros.

=> We should have a way to link bundles.  (For that matter, we should
   have a notion of shared libraries that can be loaded independently
   and with shared read-only physical memory, rather than as a bodgy
   sequence of bands copying the data around.)