fix broken thread list unlocking after fork

fix broken thread list unlocking after fork

apparently Linux clears the registered exit futex address on fork.
this means that, if after forking the child process becomes
multithreaded and the original thread exits, the thread list will
never be unlocked, and future attempts to use the thread list will
deadlock.

re-register the exit futex address after _Fork in the child to ensure
that it's preserved.