Fix __fp_lock_all() and __fp_unlock_all()

For _REENT_GLOBAL_STDIO_STREAMS, lock/unlock all FILE objects.  In the
repository, this function is only used by Cygwin during process forks.  Since
Cygwin enabled _REENT_GLOBAL_STDIO_STREAMS recently, without this fix no FILE
object at all was locked.

newlib/libc/stdio/findfp.c

@@ -329,6 +329,8 @@ __fp_lock_all (void)
 #ifndef _REENT_GLOBAL_STDIO_STREAMS
   ptr = _REENT;
   (void) _fwalk_sglue (ptr, __fp_lock, &ptr->__sglue);
+#else
+  (void) _fwalk_sglue (NULL, __fp_lock, &__sglue);
 #endif
 }
 
@@ -340,6 +342,8 @@ __fp_unlock_all (void)
 
   ptr = _REENT;
   (void) _fwalk_sglue (ptr, __fp_unlock, &ptr->__sglue);
+#else
+  (void) _fwalk_sglue (NULL, __fp_unlock, &__sglue);
 #endif
 
   __sfp_lock_release ();