A new thread exit lock and flag are introduced. They are used to
detect that the thread is about to exit or has exited in
__pthread_kill_internal, and the signal is not sent in this case.
The test sysdeps/pthread/tst-pthread_cancel-select-loop.c is derived
from a downstream test originally written by Marek Polacek.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit 526c3cf11e)
