Build Go tools for arm

Test: prebuilts/build-tools/build-prebuilts.sh
Change-Id: I915be7bb7451f27fcd4beba5e6487da7a01639a6
diff --git a/ui/build/config.go b/ui/build/config.go
index a4f778d..94b0781 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -1117,11 +1117,18 @@
 func (c *configImpl) PrebuiltOS() string {
 	switch runtime.GOOS {
 	case "linux":
-		return "linux-x86"
+		switch runtime.GOARCH {
+		case "amd64":
+			return "linux-x86"
+		case "arm64":
+			return "linux-arm64"
+		default:
+			panic(fmt.Errorf("Unknown GOARCH %s", runtime.GOARCH))
+		}
 	case "darwin":
 		return "darwin-x86"
 	default:
-		panic("Unknown GOOS")
+		panic(fmt.Errorf("Unknown GOOS %s", runtime.GOOS))
 	}
 }
 
@@ -1711,13 +1718,7 @@
 }
 
 func (c *configImpl) HostPrebuiltTag() string {
-	if runtime.GOOS == "linux" {
-		return "linux-x86"
-	} else if runtime.GOOS == "darwin" {
-		return "darwin-x86"
-	} else {
-		panic("Unsupported OS")
-	}
+	return c.PrebuiltOS()
 }
 
 func (c *configImpl) KatiBin() string {
diff --git a/ui/build/path.go b/ui/build/path.go
index cc1d7e9..b92d799 100644
--- a/ui/build/path.go
+++ b/ui/build/path.go
@@ -20,7 +20,6 @@
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 
 	"github.com/google/blueprint/microfactory"
@@ -122,7 +121,7 @@
 	myPath, _ = filepath.Abs(myPath)
 
 	// Set up the checked-in prebuilts path directory for the current host OS.
-	prebuiltsPath, _ := filepath.Abs("prebuilts/build-tools/path/" + runtime.GOOS + "-x86")
+	prebuiltsPath, _ := filepath.Abs("prebuilts/build-tools/path/" + config.PrebuiltOS())
 	myPath = prebuiltsPath + string(os.PathListSeparator) + myPath
 
 	// Set $PATH to be the directories containing the host tool symlinks, and
@@ -258,7 +257,7 @@
 
 	// We put some prebuilts in $PATH, since it's infeasible to add dependencies
 	// for all of them.
-	prebuiltsPath, _ := filepath.Abs("prebuilts/build-tools/path/" + runtime.GOOS + "-x86")
+	prebuiltsPath, _ := filepath.Abs("prebuilts/build-tools/path/" + config.PrebuiltOS())
 	myPath = prebuiltsPath + string(os.PathListSeparator) + myPath
 
 	// Replace the $PATH variable with the path_interposer symlinks, and