[Guidelines] MUST: use ExcludeArch over ExclusiveArch

[Rex Dieter]

Alex Hudson wrote:


> It doesn't make a lot of sense to me to ask Lubomir, the packager, to
> change the current ExclusiveArch, because I think that is the more
> correct expression of what this software supports. However, as a MUST:
> requirement this is a review blocker.

ExclusiveArch is a justifiable exception here (ie, to use in the case of 
something known to only build on particular architectures).

-- Rex