Fix valgrind suppression for thread pool

Summary:
Extending the fix in D28902642 to capture this leak we're seeing:

```
64 bytes in 2 blocks are still reachable in loss record 13 of 20
   at 0x932F74F: malloc (in /usr/local/fbcode/platform009/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x8B0CCC6: _dl_close_worker.part.0 (in /usr/local/fbcode/platform009/lib/ld-2.30.so)
   by 0x8B0D7 (facebook/mysql-5.6@b384483e877907c499ed3ca54cefbd807d7b77a7)FF: _dl_close (in /usr/local/fbcode/platform009/lib/ld-2.30.so)
   by 0x949ACA1: _dl_catch_exception (in /usr/local/fbcode/platform009/lib/libc-2.30.so)
   by 0x949AD34: _dl_catch_error (in /usr/local/fbcode/platform009/lib/libc-2.30.so)
   by 0x95BC991: _dlerror_run (in /usr/local/fbcode/platform009/lib/libdl-2.30.so)
   by 0x95BC343: dlclose (in /usr/local/fbcode/platform009/lib/libdl-2.30.so)
   by 0x5861EAE: free_plugin_mem(st_plugin_dl*) (sql_plugin.cc:614)
   by 0x58663AB: plugin_dl_del(MYSQL_LEX_STRING const*) (sql_plugin.cc:876)
   by 0x585FEDA: plugin_del(st_plugin_int*) (sql_plugin.cc:1146)
   by 0x585BB8F: reap_plugins() (sql_plugin.cc:1186)
   by 0x5861228: plugin_shutdown() (sql_plugin.cc:2057)
   by 0x5456C17: clean_up(bool) (mysqld.cc:2671)
   by 0x5447D1 (facebook/mysql-5.6@6f2a027016775f18faf43b7c574c251ac7242108)D: mysqld_main(int, char**) (mysqld.cc:8193)
   by 0x4F78351: main (main.cc:25)
```

Also, the symbols need to be in their mangled form to work.

Reviewed By: george-reynya

Differential Revision: D29769404

fbshipit-source-id: 350bb57e95a

mysql-test/valgrind.supp

@@ -948,11 +948,13 @@
    Memcheck:Leak
    fun:malloc
    fun:_dl_close_worker*
+   ...
    fun:_dl_close
    fun:_dl_catch_exception
+   ...
    fun:_dlerror_run
    fun:dlclose
-   fun:free_plugin_mem
+   fun:_ZL15free_plugin_memP12st_plugin_dl
    ...
 }