machine placement spec tweak

[William Reade]

How I'm actually working on this, it suddenly seems like a huge
oversight that we haven't got a constraint something like:

    arch=x86|x64|arm

...and I don't recall really discussing it (well, I remember us deciding
that we should just go with ECU, and that makes sense as far as it goes,
but architecture seems pretty fundamental too, and I'm not sure it got
the discussion it deserved).

I'm not proposing we change the spec right now, but it shouldn't be too
hard to bolt on additional constraints after the initial implementation;
so, does anyone else think this would be a good thing to add if we have
time?

Cheers
William