nptl: Fix sem_wait and sem_timedwait cancellation (BZ#18243)

This patch fixes both sem_wait and sem_timedwait cancellation point for
uncontended case.  In this scenario only atomics are involved and thus
the futex cancellable call is not issue and a pending cancellation signal
is not handled.

The fix is straighforward by calling pthread_testcancel is both function
start.  Although it would be simpler to call CANCELLATION_P directly, I
decided to add an internal pthread_testcancel alias and use it to export
less internal implementation on such function.  A possible change on
how pthread_testcancel is internally implemented would lead to either
continue to force use CANCELLATION_P or to adjust its every use.

GLIBC testcase also does have tests for uncontended cases, test-cancel12
and test-cancel14.c,  however both are flawed by adding another
cancellation point just after thread pthread_cleanup_pop:

 47 static void *
 48 tf (void *arg)
 49 {
 50   pthread_cleanup_push (cleanup, NULL);
 51
 52   int e = pthread_barrier_wait (&bar);
 53   if (e != 0 && e != PTHREAD_BARRIER_SERIAL_THREAD)
 54     {
 55       puts ("tf: 1st barrier_wait failed");
 56       exit (1);
 57     }
 58
 59   /* This call should block and be cancelable.  */
 60   sem_wait (&sem);
 61
 62   pthread_cleanup_pop (0);
 63
 64   puts ("sem_wait returned");
 65
 66   return NULL;
 67 }

So sem_{timed}wait does not act on cancellation, pthread_cleanup_pop executes
'cleanup' and then 'puts' acts on cancellation.  Since pthread_cleanup_pop
removed the clean-up handler, it will ran only once and thus it won't accuse
an error to indicate sem_wait has not acted on the cancellation signal.

This patch also fixes this behavior by removing the cancellation point 'puts'.
It also adds some cleanup on all sem_{timed}wait cancel tests.

It partially fixes BZ #18243.  Checked on x86_64.

	[BZ #18243]
	* nptl/pthreadP.h (__pthread_testcancel): Add prototype and hidden_proto.
	* nptl/pthread_testcancel.c (pthread_cancel): Add internal aliais
	definition.
	* nptl/sem_timedwait.c (sem_timedwait): Add cancellation check for
	uncontended case.
	* nptl/sem_wait.c (__new_sem_wait): Likewise.
	* nptl/tst-cancel12.c (cleanup): Remove wrong cancellation point.
	(tf): Fix check for uncontended case.
	(do_test): Likewise.
	* nptl/tst-cancel13.c (cleanup): Remove wrong cancellation point.
	(tf): Fix check for uncontended case.
	(do_test): Likewise.
	* nptl/tst-cancel14.c (cleanup): Remove wrong cancellation point.
	(tf): Fix check for uncontended case.
	(do_test): Likewise.
	* nptl/tst-cancel15.c (cleanup): Remove wrong cancellation point.
	(tf): Fix check for uncontended case.
	(do_test): Likewise.