gdbstub: add multiprocess support to 'D' packets

gdbstub: add multiprocess support to 'D' packets

'D' packets are used by GDB to detach from a process. In multiprocess
mode, the PID to detach from is sent in the request.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20181207090135.7651-11-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>