gdbstub: Replace two find_cpu()s with qemu_get_cpu()

Since commit cb446ecab714b2444a270be209e0533bcd2ee534 (kvm: Change
cpu_synchronize_state() argument to CPUState), one was no longer
accessing CPUArchState, the other was just checking existence.

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