x86: Initialize stack canary in secondary start

Some secondary clockevent setup code needs to call request_irq, which
will cause fake stack check failure in schedule() if voluntary
preemption model is chosen.  It is safe to have stack canary
initialized here early, since start_secondary() does not return.

Signed-off-by: Jacob Pan <jacob.jun.pan@intel.com>
LKML-Reference: <43F901BD926A4E43B106BF17856F07559FB80D02@orsmsx508.amr.corp.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>