[gn] Enable stricter build flags

src/interpreter/bytecodes.h

Index: src/interpreter/bytecodes.h
diff --git a/src/interpreter/bytecodes.h b/src/interpreter/bytecodes.h
index f80469f523c4722f56d1b864d795ba8bacc65cff..8920440768a8f77d1b9d560597ae3d6e15714981 100644
--- a/src/interpreter/bytecodes.h
+++ b/src/interpreter/bytecodes.h
@@ -789,6 +789,7 @@ class V8_EXPORT_PRIVATE Bytecodes final {
       default:
         return 0;
     }
+    UNREACHABLE();
     return 0;
   }