The problem was located to lie inside current NPTL pthread_exit() 
implementation. Race conditions in this code can lead to segmentation
fault. Hovewer, this can happen only in a race between first thread 
calling pthread_exit() and other threads. 

Workaround implemented in this patch spawns a dummy thread, which
exits immediately, during thread lib initialization. This will exclude
segment violations when further threads exit.
 


include/my_pthread.h:
  define macro NPTL_PTHREAD_EXIT_BUG which controls whether workaround
  in my_thread_global_init() is included or not.
mysys/my_thr_init.c:
  Spawn a dummy thread in my_thread_global_init() to initialize pthread 
  lib internal variables.