support target-driven cross-compilation

Running ./make-config.sh --host-location="host:path/to/sbcl" on the
target causes the make-target-N.sh scripts to do all the copying of
files to and from the host using rsync on the target to the specified
host location.

Because this is rsync, managing the reverse ssh to do this on
heavily-firewalled targets is not straightforward; we might have to
extend the syntax, or have an extra --host-port argument.  But this
should I think make the port to arm64 more straightforward.
force use of arm instruction set for building the runtime on ARM

(That's "as opposed to Thumb", which generally interworks fine
but which our build tools have a certain amount of difficulty
with.)

Pushed from my BeagleBone