machine placement spec tweak

[William Reade]

On Wed, 2011-12-14 at 11:06 -0200, Gustavo Niemeyer wrote:
> > 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
> 
> Oh, indeed! I'm pretty sure we actually debated this at UDS.
> 
> Please use amd64/386/arm

Sounds good. Unset by default?

> > I'm not proposing we change the spec right now, but it shouldn't be too
> 
> I suggest we do change it.

Well, since you're twisting my arm :p.

Cheers
William