Revert "gdbstub: Simplify find_cpu()"

This reverts commit c52a6b67, which
replaced cpu_index() with cpu_index field, leading to deviation from
thread ID for NTPL and off-by-one otherwise.

Reported-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>