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 data
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.

Closes #30898.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>