> I see no such requirements in GPLv2, GPLv3 or any other sane license.
It's not a legal requirement, but having a non-broken build system (which includes cross build capability) is a part of releasing open source quality software.
> This may break native compilation instead
Then test native compilation too once in a release cycle.
> Sure. As long as ARM remains “an embedded architecture”. But in this case it makes little sense to promote it to “primary architecture” status
Embedded architectures can be extremely important, since everyone uses a cell phone for instance; that doesn't mean it's a good idea to use them as build farm servers.
Maybe ARM servers will be viable eventually, but definitely not now.
For instance according to Wikipedia, Cortex A15 has 1/3 the Dhrystone IPC of Core i7 2600k, which isn't a good sign, although it could perhaps still win on system price/performance (kind of unlikely though).