partially reverts https://github.com/protocolbuffers/protobuf/pull/23378
to prevent error ld.lld: error: undefined symbol: __stop_linkarr_upb_AllExts

--- a/upb/port/def.inc
+++ b/upb/port/def.inc
@@ -511,7 +511,7 @@ Error, UINTPTR_MAX is undefined
 #if defined(__ELF__) || defined(__wasm__)
 
 #define UPB_LINKARR_APPEND(name) \
-  __attribute__((section("linkarr_" #name))) UPB_NO_SANITIZE_ADDRESS
+  __attribute__((retain, used, section("linkarr_" #name))) UPB_NO_SANITIZE_ADDRESS
 #define UPB_LINKARR_DECLARE(name, type) \
   extern type __start_linkarr_##name;   \
   extern type __stop_linkarr_##name;    \