title: Go on DragonFly BSD

From https://github.com/golang/go/issues/34958#issuecomment-543852995 :

Go's DragonFly support policy is that we support the latest stable release primarily, but also try to keep DragonFly master passing, in prep for it to become the latest stable release.

But that does mean we need one more builder at the moment.

Kernel versionArchitecturesInitial support versionFinal support version
4.6 or aboveamd64Go 1.8
4.4.4 (EOL)amd64Go 1.8
4.4 through 4.4.3 (EOL)amd64Go 1.3 *Go 1.7.6
3.8 through 4.2 (EOL)amd64Go 1.3 *Go 1.7.6
3.6 (EOL)amd64, 386Go 1.3Go 1.4.3

* Go 1.5 or above is recommended.