I think I may have found the problem: https://github.com/golang/go/issues/20009#issuecomment-294437962
It looks like the ports machines building Go probably have SSE/SSE2, so the resulting binary expects those to be present. I'm not sure what can be done short of building Go on my Soekris… I can't fault anyone for using a more "modern" CPU when building the ports tree for distribution.