tool/gocross: embed the version explicitly with linker flags

We need to build gocross from multiple repos, but Go's innate
git hash embedding only works when you build gocross from this repo,
not when you build it from elsewhere via 'go build
tailscale.com/tool/gocross'. Instead, explicitly embed the version
found with 'git rev-parse HEAD', which will work from any git repo.

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

tool/gocross/gocross.go

@@ -16,9 +16,9 @@
 	"fmt"
 	"os"
 	"path/filepath"
-	runtimeDebug "runtime/debug"
 
 	"tailscale.com/atomicfile"
+	"tailscale.com/version"
 )
 
 func main() {
@@ -29,12 +29,7 @@ func main() {
 		// any time.
 		switch os.Args[1] {
 		case "gocross-version":
-			hash, err := embeddedCommit()
-			if err != nil {
-				fmt.Fprintf(os.Stderr, "getting commit hash: %v", err)
-				os.Exit(1)
-			}
-			fmt.Println(hash)
+			fmt.Println(version.GetMeta().GitCommit)
 			os.Exit(0)
 		case "is-gocross":
 			// This subcommand exits with an error code when called on a
@@ -132,16 +127,3 @@ func debug(format string, args ...interface{}) {
 
 	fmt.Fprintf(out, format, args...)
 }
-
-func embeddedCommit() (string, error) {
-	bi, ok := runtimeDebug.ReadBuildInfo()
-	if !ok {
-		return "", fmt.Errorf("no build info")
-	}
-	for _, s := range bi.Settings {
-		if s.Key == "vcs.revision" {
-			return s.Value, nil
-		}
-	}
-	return "", fmt.Errorf("no git commit found")
-}