gdbstub: don't complain about preemptive ACK chars

When starting a remote connection GDB sends an '+':

  /* Ack any packet which the remote side has already sent.  */
  remote_serial_write ("+", 1);

which gets flagged as a garbage character in the gdbstub state
machine. As gdb does send it out lets be permissive about the handling
so we can better see real issues.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: gdb-patches@sourceware.org
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230810153640.1879717-9-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

gdbstub/gdbstub.c

@@ -2074,6 +2074,11 @@ void gdb_read_byte(uint8_t ch)
                 gdbserver_state.line_buf_index = 0;
                 gdbserver_state.line_sum = 0;
                 gdbserver_state.state = RS_GETLINE;
+            } else if (ch == '+') {
+                /*
+                 * do nothing, gdb may preemptively send out ACKs on
+                 * initial connection
+                 */
             } else {
                 trace_gdbstub_err_garbage(ch);
             }