[MLton] x86_64 port status

[Stephen Weeks]

> 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?  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).