[MLton] x86_64 branch portability

[Matthew Fluet]

> Building the new runtime requires a working `mlton' compiler.  This is
> not a problem for AIX and HP-UX, but it seems to me that for a
> completely new port it _is_ a problem for obvious reasons...  Am I
> missing something?

You're only missing the fact that generating basis-ffi.h and basis-ffi.sml 
from basis-ffi.def is platform independent; so, you can easily generate 
the files on one platform and copy them to the new platform.

In fact, basis-ffi.def has pretty much stopped changing, so it probably 
makes sense to keep basis-ffi.h and basis-ffi.sml in the SVN repository 
and add a .PHONY makefile rule to regenerate them when needed.

> Also, it `bin/add-cross' is not working for me.

I never use the cross compilation scripts.