archs38: mark as source-only
archs38 seems to be pretty much unused, usually only treewide changes or
kernel bumps in order to branch off new stable are done to it.
Considering that target only support some Synopsis HS38 ARC reference
boards and no consumer hardware so mark the target as source-only to stop
using Buildbot resources on building the target and packages for it.
Signed-off-by: Robert Marko <[email protected]>
Robert Marko