runtime/cgo: ignore unknown warning options

For #65290
Fixes #65971

Change-Id: If15853f287e06b85bb1cb038b3785516d5812f84
Reviewed-on: https://go-review.googlesource.com/c/go/+/567556
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/runtime/cgo/gcc_libinit.c

@@ -8,6 +8,7 @@
 // and cause a compiler warning. This results in a build failure since
 // cgo uses -Werror. See #65290.
 #pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Wunknown-warning-option"
 #pragma GCC diagnostic ignored "-Watomic-alignment"
 
 #include <pthread.h>