the attached patch fixes the scheduler's migration thread startup bug that
got unearthed by Rusty's recent CPU-startup enhancements.

the fix is to let a startup-helper thread migrate the migration thread,
instead of the migration thread calling set_cpus_allowed() itself.
Migrating a not running thread is a simple and robust thing, and needs no
cooperation from migration threads - thus the catch-22 problem of how to
migrate the migration threads is solved finally.

the patch is against Rusty's initcall fix/hack which calls
migration_init() before other CPUs are brought up - this ordering is
clearly the clean way of doing migration init. [the patch also fixes a UP
compiliation bug in Rusty's hack.]