[MLton] Small bug in build script using sml/nj on freebsd

[Matthew Fluet]

> When building mlton using sml/nj on freebsd the file
> on gets the file
> 
>      build/lib/mlton.x86-bsd
> 
> but the the script
> 
>      build/bin/mlton
> 
> expects it to have the name
> 
>      build/lib/mlton.x86-freebsd
> 
> (Note: I used the tar.gz from 2004-11-09)

I checked in a minor change to the mlton-script script to try to use 
SML/NJ's .arch-n-opsys to figure out the right heap suffix.