Handle arch/os-specific product variables

Bug: 183595873
Test: go test bp2build tests
Change-Id: I36e93ae1eb2943555dd304d5bdf62d995e77b437
diff --git a/bp2build/configurability.go b/bp2build/configurability.go
index 3cdc994..9869c5d 100644
--- a/bp2build/configurability.go
+++ b/bp2build/configurability.go
@@ -34,7 +34,7 @@
 		selectValues = append(selectValues, osSelects)
 	}
 
-	for _, pv := range list.ProductValues {
+	for _, pv := range list.SortedProductVariables() {
 		s := make(selects)
 		if len(pv.Values) > 0 {
 			s[pv.SelectKey()] = reflect.ValueOf(pv.Values)