cmd/go: add GOOS/GOARCH to go version output

Fixes #4492.

% go version
go version devel +6b602ab487d6 Sat Dec 08 14:43:00 2012 +0100 linux/amd64

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6906058

src/cmd/go/version.go

@@ -21,5 +21,5 @@ func runVersion(cmd *Command, args []string) {
 		cmd.Usage()
 	}
 
-	fmt.Printf("go version %s\n", runtime.Version())
+	fmt.Printf("go version %s %s/%s\n", runtime.Version(), runtime.GOOS, runtime.GOARCH)
 }