libgfortran: Bugfix if not define HAVE_ATOMIC_FETCH_ADD

This patch try to fix the bug when HAVE_ATOMIC_FETCH_ADD is
not defined in dec_waiting_unlocked function. As io.h does
not include async.h, the WRLOCK and RWUNLOCK macros are
undefined.

libgfortran/ChangeLog:

	* io/io.h (dec_waiting_unlocked): Use
	__gthread_rwlock_wrlock/__gthread_rwlock_unlock or
	__gthread_mutex_lock/__gthread_mutex_unlock functions
	to replace WRLOCK and RWUNLOCK macros.

Signed-off-by: Lipeng Zhu <lipeng.zhu@intel.com>