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 Homebrew/homebrew#30898.

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

Formula/go.rb

@@ -26,10 +26,13 @@ class Go < Formula
     if build.include? 'cross-compile-all'
       targets = [
         ['linux',   ['386', 'amd64', 'arm']],
-        ['freebsd', ['386', 'amd64']],
-        ['netbsd',  ['386', 'amd64']],
+        ['freebsd', ['386', 'amd64', 'arm']],
+        ['netbsd',  ['386', 'amd64', 'arm']],
         ['openbsd', ['386', 'amd64']],
         ['windows', ['386', 'amd64']],
+        ['dragonfly', ['386', 'amd64']],
+        ['plan9',   ['386', 'amd64']],
+        ['solaris', ['amd64']],
         ['darwin',  ['386', 'amd64']],
       ]
     elsif build.include? 'cross-compile-common'