dashboard: enable linux/loong64 builder on x/arch

A patch (CL 358854) implementing the loong64 GNU format disassembler on x/arch
has been uploaded, and the linux/loong64 builders seem to have enough resources
to handle it.

Change-Id: Ibe4d16e544343a4056ff9bd6b8ae999c4b8b6464
Reviewed-on: https://go-review.googlesource.com/c/build/+/465475
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Carlos Amedee <carlos@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: xiaodong liu <teaofmoli@gmail.com>

dashboard/builders.go

@@ -2754,7 +2754,7 @@ func init() {
 			switch repo {
 			case "go":
 				return atLeastGo1(goBranch, 19)
-			case "net", "sys":
+			case "arch", "net", "sys":
 				return branch == "master" && atLeastGo1(goBranch, 19)
 			default:
 				return false