[api] Use v8_flags for accessing flag values

Avoid the deprecated FLAG_* syntax, access flag values via the
{v8_flags} struct instead.

R=jkummerow@chromium.org

Bug: v8:12887
Change-Id: Ia17d668b3ddcbcb7a35388231aa5d80e8e5b419b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899122
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83270}

src/api/api-natives.cc

@@ -529,7 +529,7 @@ MaybeHandle<JSFunction> InstantiateFunction(
   if (!data->needs_access_check() &&
       data->GetNamedPropertyHandler().IsUndefined(isolate) &&
       data->GetIndexedPropertyHandler().IsUndefined(isolate)) {
-    function_type = FLAG_embedder_instance_types && data->HasInstanceType()
+    function_type = v8_flags.embedder_instance_types && data->HasInstanceType()
                         ? static_cast<InstanceType>(data->InstanceType())
                         : JS_API_OBJECT_TYPE;
   }