Build Go tools for arm

Test: prebuilts/build-tools/build-prebuilts.sh
Change-Id: I915be7bb7451f27fcd4beba5e6487da7a01639a6
diff --git a/android/config.go b/android/config.go
index f6d08b8..54c8aac 100644
--- a/android/config.go
+++ b/android/config.go
@@ -814,11 +814,18 @@
 func (c *config) 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))
 	}
 }