gdbstub: Change GDBState::query_cpu to CPUState

Since first_cpu/next_cpu are CPUState, CPUArchState is no longer needed.

This resolves a NULL pointer dereference of query_cpu, introduced
with commit 182735efaf956ccab50b6d74a4fed163e0f35660 and reported by
TeLeMan and Max Filippov.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>
