Change kbdb's kthr::cpu field into an int, to avoid gcc warnings about
comparing it with NOCPU, which became -1 recently.  While here, avoid
using it for address calculations if it is negative.

Reviewed by:	jhb, adrian
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274391 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f