libgomp: Don't access gomp_sem_t as int using atomics unconditionally

This patch introduces gomp_sem_getcount wrapper, which uses sem_getvalue
for POSIX and atomic loads for linux futex and accel.  rtems for now
remains broken.

2021-01-18  Jakub Jelinek  <jakub@redhat.com>

	* config/linux/sem.h (gomp_sem_getcount): New function.
	* config/posix/sem.h (gomp_sem_getcount): New function.
	* config/posix/sem.c (gomp_sem_getcount): New function.
	* config/accel/sem.h (gomp_sem_getcount): New function.
	* task.c (task_fulfilled_p): Use gomp_sem_getcount.
	(omp_fulfill_event): Likewise.