[MLton] x86-64 port

[Matthew Fluet]

> Can the x86-64 port of MLton

Well, no such beast exists currently.

> be compiled from full 64-bit mode or does it have to be bootstrapped by 
> cross-compiling from 64-bit mode with 32-bit compatibility enabled?

The plan will be to have a self-compiling 64-bit port.  Ultimately, the 
first bootstrap will come from either a 32-bit compiler cross-compiling to 
a 64-bit target, or such a 32-bit compiler running on a 64-bit platform 
with 32-bit compatibility enabled, but once one such bootstrap has been 
released, future releases should be compiled by previous releases.