[MLton] x86_64 port status

[Stephen Weeks]

> Wouldn't it be better to put it in the appropriate platform specific
> header file? 

I agree.  I would certainly put the definition in solaris.h.

Relevant to that, I noticed that gen-types.c included cenv.h rather
than platform.h.  I was wondering why.  For Solaris it caused me to
duplicate a couple of #ifs that were nicely handled by the platform/*
stuff.  It seems like it would be better to include platform.h if at
all possible.