[MLton] x86_64 port status

[Matthew Fluet]

>>>  #ifndef PRIxPTR
>>>  #define PRIxPTR "x"
>>>  #endif
>>
>> That was my intention, but as you note, it may not be correct for all
>> configurations.
>
> What do you think of my suggestion of automatically defining PRIxPTR
> in c-types.h?

That's reasonable, although I think it is equally reasonable to use 
platform/<os>.h.  The auto-generation is convenient, but I think that it 
will very soon quiesce, and we should be interested in situations where 
the auto-generated sizes don't equal the sizes we've seen on other 
instances of that platform.  In other words, do you really think it 
reasonable to expect sparc-solaris to work unmodified on 32-bit sparcs and 
64-bit sparcs?