tsan: fix failures after multi-threaded fork

Creating threads after a multi-threaded fork is semi-supported,
we don't give particular guarantees, but we try to not fail
on simple cases and we have die_after_fork=0 flag that enables
not dying on creation of threads after a multi-threaded fork.
This flag is used in the wild:
https://github.com/mongodb/mongo/blob/23c052e3e321dbab90f1863d4d5539d7c1a1cf44/SConstruct#L3599

fork_multithreaded.cpp test started hanging in debug mode
after the recent "tsan: fix deadlock during race reporting" commit,
which added proactive ThreadRegistryLock check in SlotLock.

But the test broke earlier after "tsan: remove quadratic behavior in pthread_join"
commit which made tracking of alive threads based on pthread_t stricter
(CHECK-fail on 2 threads with the same pthread_t, or joining a non-existent thread).
When we start a thread after a multi-threaded fork, the new pthread_t
can actually match one of existing values (for threads that don't exist anymore).
Thread creation sta...