Fix 324227 memcheck false positive leak when a thread calls exit+block

Fix 324227  memcheck false positive leak when a thread calls exit+block
only reachable via other thread live register

The exiting thread will have its registers considered as not reachable
anymore, registers of other threads will be considered reachable.

This is ensured by using a different exit reason for the
exiting thread and for the other threads.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13670