Update the dg-skip to cover newer systems.
gcc/testsuite/ChangeLog:
* obj-c++.dg/objc-gc-3.mm:i Skip for Darwin >= 16.
/* Contributed by Ziemowit Laski <zlaski@apple.com> */
/* { dg-do compile } */
-/* { dg-skip-if "GC API is an error from Darwin16." { *-*-darwin1[6-8]* } { "-fnext-runtime" } { "" } } */
+/* { dg-skip-if "GC API is an error from Darwin16." { *-*-darwin1[6-9]* *-*-darwin2[0-9]* } { "-fnext-runtime" } { "" } } */
/* { dg-options "-fobjc-gc" } */
/* { dg-prune-output "cc1objplus: warning: '-fobjc-gc' is ignored for '-fgnu-runtime'" } */