[MLton] starting builds

[Stephen Weeks]

> The point is that on some architectures you will have to.
> I suppose this can be deferred until MLton is ported to them.
> 
> For example, arm can take the floor, but has no concept of
> a rounding mode in the processor. The ML basis requires a
> working floor (as does C) on these platforms, but both allow
> for rounding modes to be missing on some platforms.

OK.  Then let's defer fixing the problem until we port to such a
platform, when we will have both the motivation to fix it and be in
the situation to better test that the fix works.