newlib: Unlock the mutex while calling atexit()'ed functions

The atexit()'ed function may deadlock if it waits for another thread
which calls atexit() in the current __call_atexit.c code. This patch
unlock __atexit_recursive_mutex while calling atexit()'ed functions
to avoid the deadlock mentioned above. glibc and Darwin do the same,
so it sounds reasonable.

Addresses: https://cygwin.com/pipermail/cygwin/2025-October/258930.html
Reported-by: Tomohiro Kashiwada <tomohiro-kashiwada@ezweb.ne.jp>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
(cherry picked from commit a2a8bc7)

Author: tyan0

newlib/libc/stdlib/__call_atexit.c

@@ -114,13 +114,21 @@ __call_exitprocs (int code, void *d)
 
 	  ind = p->_ind;
 
+#ifndef __SINGLE_THREAD__
+	  /* Unlock __atexit_recursive_mutex; otherwise, the function fn() may
+	     deadlock if it waits for another thread which calls atexit(). */
+	  __lock_release_recursive(__atexit_recursive_mutex);
+#endif
 	  /* Call the function.  */
 	  if (!args || (args->_fntypes & i) == 0)
 	    fn ();
 	  else if ((args->_is_cxa & i) == 0)
 	    (*((void (*)(int, void *)) fn))(code, args->_fnargs[n]);
 	  else
 	    (*((void (*)(void *)) fn))(args->_fnargs[n]);
+#ifndef __SINGLE_THREAD__
+	  __lock_acquire_recursive(__atexit_recursive_mutex);
+#endif
 
 	  /* The function we called call atexit and registered another
 	     function (or functions).  Call these new functions before