Uthread blockon handles notif_disabled uthreads

Uthread blockon handles notif_disabled uthreads

The in_vcore_context() check tests TLS, not whether or not notifs are
disabled.  It is the latter we care about more.  We need to check
DONT_MIGRATE before checking if notifs are disabled (vcoreid), and any
code that does have DONT_MIGRATE set *when making a syscall* needs to
have notifs disabled.  I don't explicitly check for this, since you have
to set DONT_MIGRATE to be sure about your vcoreid.