htl: Make sem_*wait cancellations points

By aligning its implementation on pthread_cond_wait.

* sysdeps/htl/sem-timedwait.c (cancel_ctx): New structure.
(cancel_hook): New function.
(__sem_timedwait_internal): Check for cancellation and register
cancellation hook that wakes the thread up, and check again for
cancellation on exit.
* nptl/tst-cancel13.c, nptl/tst-cancelx13.c: Move to...
* sysdeps/pthread/: ... here.
* nptl/Makefile: Move corresponding references and rules to...
* sysdeps/pthread/Makefile: ... here.