gdbstub: don't complain about preemptive ACK chars

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>