gdbstub: more fixes for client Ctrl-C handling

gdbstub: more fixes for client Ctrl-C handling

The original fix caused problems with spurious characters on other
system emulation. So:

  - instead of spamming output make the warning a trace point
  - ensure we only allow a stop reply if it was 0x3

Suggested-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <456ed3318421dd7946bdfb5ceda7e05332da368c.1690910333.git.quic_mathbern@quicinc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230810153640.1879717-8-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>