go: Add latest available cross-compilers

Recent releases of Go have added support for additional platforms, so the
--cross-compile-all command of this formula has gotten out of date relative to
"all" available cross-compilers. This update adds on the ARM architecture to
some existing platforms, plus the Solaris, Plan9, and Dragonfly BSD platforms
that are supported in Go 1.3. (--cross-compile-common remains unchanged)