gdbstub: add multiprocess support to 'sC' packets

gdbstub: add multiprocess support to 'sC' packets

Change the sC packet handling to support the multiprocess extension.
Instead of returning the first thread, we return the first thread of the
current process.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181207090135.7651-7-luc.michel@greensocs.com
[PMM: corrected checkpatch comment style nit]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>