This makes the nptl/tst-sem6 interrupt sem_wait reliably by letting the
signal handler call sem_post.  This ensures that if the signal handler
should happen to run before sem_wait actually starts, sem_wait will find
an available token and return.  This is necessary if a program does not
want to rely on timing nor on forward progress / fairness guarantees of
the OS scheduler.

It's needed so that Patch 3/3 can do the right thing in face of a
FUTEX_WAIT that may return EINTR on either signal interruption or
spuriously.