gdbstub: gdb_set_stop_cpu: ignore request when process is not attached

gdbstub: gdb_set_stop_cpu: ignore request when process is not attached

When gdb_set_stop_cpu() is called with a CPU associated to a process
currently not attached by the GDB client, return without modifying the
stop CPU. Otherwise, GDB gets confused if it receives packets with a
thread-id it does not know about.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181207090135.7651-15-luc.michel@greensocs.com
[PMM: fix checkpatch comment style nit]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>