[MLton] add-cross no longer uses a cross compiler

[Stephen Weeks]

> Is there any way to retain the old method - with a flag to the
> script or something?

Yes, we can do this if you need.  If you are unable to work with the
new script, feel free to use the old one, and once you get a setup
that works, mail it and I will add it back via a switch.  But it
sounds like from your other mails that you might be able to get a gcc
and ssh working on Windows/MinGW.

> And even worse, what if people don't have more than one machine?

I would guess that if anyone is doing cross development, then they
must have the machine they are compiling to so they can test their
program.  In any case, the old script suffered from the same problems
in that it required to be able to ssh to the target machine and run a
program.  The new problem that this script adds is that it requires a
gcc on the target machine.

To be clear, I have not changed (and am not proposing to change) how
people use MLton for cross development.  They can still use a cross
gcc if they want.  What I have changed is how people build MLton for
new targets.  I have eliminated the the requirement for a cross gcc
and added a requirement for a native gcc on the target.  Overall, I
think this is a win due to the complexities of cross gcc's.