[gdb] Handle pending ^C after rl_callback_read_char

In completion tests in various test-cases, we've been running into these
"clearing input line" timeouts:
...
(gdb) $cmd^GPASS: gdb.gdb/unittest.exp: tab complete "$cmd"
FAIL: gdb.gdb/unittest.exp: tab complete "$cmd" (clearing input line) (timeout)
...
where $cmd == "maintenance selftest name_that_does_not_exist".

AFAIU, the following scenario happens:
- expect sends "$cmd\t"
- gdb detects the stdin event, and calls rl_callback_read_char until it
  comes to handle \t
- readline interprets the \t as completion, tries to complete, fails to do so,
  outputs a bell (^G)
- expect sees the bell, and proceeds to send ^C
- readline is still in the call to rl_callback_read_char, and stores the
  signal in _rl_caught_signal
- readline returns from the call to rl_callback_read_char, without having
  handled _rl_caught_signal
- gdb goes to wait for the next event
- expect times out waiting for "Quit", the expected reaction for ^C

Fix this by handling pending signals after each call to rl_callback_read_char.

The fix is only available for readline 8.x, if --with-system-readline provides
an older version, then the fix is disabled due to missing function
rl_check_signals.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27813

gdb/event-top.c

@@ -186,6 +186,22 @@ gdb_rl_callback_read_char_wrapper_noexcept () noexcept
   TRY_SJLJ
     {
       rl_callback_read_char ();
+#if RL_VERSION_MAJOR >= 8
+      /* It can happen that readline (while in rl_callback_read_char)
+	 received a signal, but didn't handle it yet.  Make sure it's handled
+	 now.  If we don't do that we run into two related problems:
+	 - we have to wait for another event triggering
+	   rl_callback_read_char before the signal is handled
+	 - there's no guarantee that the signal will be processed before the
+	   event.  */
+      while (rl_pending_signal () != 0)
+	/* Do this in a while loop, in case rl_check_signals also leaves a
+	   pending signal.  I'm not sure if that's possible, but it seems
+	   better to handle the scenario than to assert.  */
+	rl_check_signals ();
+#else
+      /* Unfortunately, rl_check_signals is not available.  */
+#endif
       if (after_char_processing_hook)
 	(*after_char_processing_hook) ();
     }