libgfortran: Use `__gthread_t` instead of `pthread_t`

It used to cause errors if a thread model other than `posix` was selected,
which looks like a leftover from a7987858.

libgfortran/
	* io/async.h (struct async_unit): Use `__gthread_t` instead
	of `pthread_t`.

Signed-off-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>

libgfortran/io/async.h

@@ -351,7 +351,7 @@ typedef struct async_unit
   struct adv_cond work;
   struct adv_cond emptysignal;
   struct st_parameter_dt *pdt;
-  pthread_t thread;
+  __gthread_t thread;
   struct transfer_queue *head;
   struct transfer_queue *tail;