tst-backtrace5: Fix on hurd

hurd has intermediate io_read and read_nocancel functions, skip them down to
the actual read call from the test.

debug/tst-backtrace5.c

@@ -77,9 +77,9 @@ handle_signal (int signum)
 
   /* Skip the signal trampoline and any cancellable syscall wrapper frames
      (__syscall_cancel*) and require the read syscall wrapper to be
-     present.  */
+     present (but skip intermediates).  */
   for (i = 1; i < n; i++)
-    if (match (symbols[i], "read"))
+    if (match (symbols[i], "read") && !match (symbols[i], "read_nocancel") && !match (symbols[i], "io_read"))
       break;
   if (i == n)
     {