I think you are doing it wrong, as you are telling go to run the build script on ARM (which you are not running), not to compile syncthing to arm. You should do:
GOARM=5 go run build.go -goos linux -goarch arm, or perhaps even modify build.go to support GOARM.
BTW how are you building the official ARM binaries? Like this.
Because this build works for me, and the only reason that I am trying to compile Syncthing by myself, because my NAS has a NAND chip where it stores its firmware and the complete FHS comes from a mounted .img file. So the whole system is read only. But there is a symlink from my hard drive to /ffp where I have a complete FHS with r/w permissions and a lot of self compiled things. And I want to adapt the pathes in the Syncthing source and build a package from that. This is the goal so far.