We can't call kstat_create(9) when bringing up the secondary CPUs as it

We can't call kstat_create(9) when bringing up the secondary CPUs as it
uses an rwlock and curproc isn't initialized yet for these CPUs at this
point.  As a result we hit a "locking against myself" panic if there is
any lock contention.

Fix this by adding a new ci_midr member to struct cpu_info which gets
initialized when we identify the CPUs and use that to attach the kstat
stuff.

ok tobhe@, dlg@