[MLton] x86_64 branch portability

[Stephen Weeks]

> Also, it `bin/add-cross' is not working for me.  It outputs these
> errors:
> 
>   runtime.c:2759: warning: redundant redeclaration of 'gcState'
...

You must have an out-of-date repository.  That bug was fixed in r4660
a couple of days ago.  add-cross used to use the COMPILE_FAST option
when compiling the runtime, which was a hack that cat'd all the
runtime C files into a single C file for (much) faster compilation.
But COMPILE_FAST is broken in the new runtime so I turned it off in
add-cross.  I don't think COMPILE_FAST is easy to fix or even worth
fixing, so it should probably be removed from runtime/Makefile.

I just used the latest add-cross today to build a
x86/Linux->Sparc/Solaris MLton, so it works as AFAIK.