oacc-init.c (acc_device_lock): Add explanatory comment.

oacc-init.c (acc_device_lock): Add explanatory comment.

	* oacc-init.c (acc_device_lock): Add explanatory comment.
	(resolve_device): Add comment about locking requirement.
	(acc_init_1, acc_shutdown_1): Likewise. Add locking around
	gomp_init_device and gomp_fini_device calls.
	(acc_get_num_devices, acc_set_device_type, acc_get_device_type)
	(acc_get_device_num, acc_set_device_num): Add locking around
	resolve_device and gomp_init_device calls.

From-SVN: r222862