gdbstub: Simplify gdb_get_cpu_pid() to use cpu->cluster_index

gdbstub: Simplify gdb_get_cpu_pid() to use cpu->cluster_index

Now we're keeping the cluster index in the CPUState, we don't
need to jump through hoops in gdb_get_cpu_pid() to find the
associated cluster object.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20190121152218.9592-5-peter.maydell@linaro.org