MDEV-35574 remove obsolete pthread_exit calls

Threads can normally exit without a explicit pthread_exit call.

There seem to date to old glibc bugs, many around 2.2.5.

The semi related bug was https://bugs.mysql.com/bug.php?id=82886.

To improve safety in the signal handlers DBUG_* code was removed.

These where also needed to avoid some MSAN unresolved stack issues.

This is effectively a backport of 2719cc4925.

storage/mroonga/vendor/groonga/src/groonga_benchmark.c

@@ -271,12 +271,7 @@ error_exit_in_thread(intptr_t code)
   CRITICAL_SECTION_ENTER(grntest_cs);
   grntest_stop_flag = 1;
   CRITICAL_SECTION_LEAVE(grntest_cs);
-#ifdef WIN32
-  _endthreadex(code);
-#else
-  pthread_exit((void *)code);
-#endif /* WIN32 */
-  return 0;
+  return code;
 }