[MLton] x86_64 port status

[Matthew Fluet]

>> I agree that it makes the most sense to move the n-way platform-os #ifdef
>> into cenv.h; I'd say move the n-way platform-arch #ifdef there as well.
>
> I did the platform-os one.  I wasn't sure about the platform-arch one.
> Did you mean the one that defines MLton_Platform_Arch_host?

Yes.

> Hmmm, I
> wonder if each of those branches should become a platform/<arch>.h
> file (that for starters would only define MLton_Platform_Arch_host).

That would seem to make some sense.