Treat stl: "system" the same way as default.

Test: verify that adding `stl: "system"` does not change the module
build.

Change-Id: I55fbde223e4a4695df7524213e1bf3671da84e8e
diff --git a/cc/stl.go b/cc/stl.go
index 5e61e1e..e59f677 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -66,7 +66,7 @@
 		}
 		if ctx.useSdk() && ctx.Device() {
 			switch s {
-			case "":
+			case "", "system":
 				return "ndk_system"
 			case "c++_shared", "c++_static":
 				return "ndk_lib" + s