gdbstub: Clean up process_string_cmd

gdbstub: Clean up process_string_cmd

Change 'process_string_cmd' to return true on success and false on
failure, instead of 0 and -1.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240628050850.536447-2-gustavo.romero@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240705084047.857176-31-alex.bennee@linaro.org>