[MLton] x86_64 port status

[Matthew Fluet]

>> What happened to fixedGetrusage, which used to live in
>> platform/linux.c (among other places), and was needed to work around
>> some old Linux kernel bugs?
>
> I think I dropped it for no better reason than it wasn't an issue on my 
> platform and it wasn't clear how long we intended to support such old broken 
> kernels.  If you want to restore it, you can easily figure out its previous 
> uses from HEAD.

In general, this is another instance where trying to build against the 
intersection of all supported platforms negatively impacts more modern, 
feature-rich platforms.