tool/gocross: add command to print the wrapper shell script

So that when importing and using gocross from other repos, there's
an easy way to get at the right wrapper script that's in sync with
the gocross binary.

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/gocross.go

@@ -56,6 +56,9 @@ func main() {
 			}
 			fmt.Println(filepath.Join(toolchain, "bin/go"))
 			os.Exit(0)
+		case "gocross-print-wrapper-script":
+			fmt.Println(wrapperScript)
+			os.Exit(0)
 		}
 	}
 
@@ -94,6 +97,9 @@ func main() {
 	doExec(filepath.Join(toolchain, "bin/go"), args, os.Environ())
 }
 
+//go:embed gocross-wrapper.sh
+var wrapperScript string
+
 func debug(format string, args ...interface{}) {
 	debug := os.Getenv("GOCROSS_DEBUG")
 	var (