[ext-code-space] Fix issues with recent shared GC changes

Bug: v8:11880
Change-Id: Ifcbf73a68c68b2534bb0c7272be43269b0963507
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3329799
Auto-Submit: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78344}

src/heap/mark-compact.cc

@@ -3254,7 +3254,7 @@ class ClientHeapVerifier final : public ObjectVisitorWithCageBases {
   }
 
   void VisitCodePointer(HeapObject host, CodeObjectSlot slot) override {
-    UNREACHABLE();
+    VerifySlot(code_cage_base(), ObjectSlot(slot.address()));
   }
 
   void VisitCodeTarget(Code host, RelocInfo* rinfo) override {}