https://bugzilla.redhat.com/show_bug.cgi?id=2215174
--- Comment #6 from Jeremy Newton alexjnewt@fastmail.com ---
But if you use noarch, I think you should only apply noarch to the devel subpackage to tell koji to specifically build it on a x86_64 machine only. I'm not 100% sure if this is correct.
I re-read the guidelines page. I think you can just set noarch and exclusive arch in the same place and this should work. There's no need to make devel package specifically noarch if I understand correctly.