Impact: misc udpate

* wrap content with CONFIG_CC_STACK_PROTECTOR so that other arch files
  can include it directly

* add missing includes

This will help future changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: x86_64 percpu area layout change, irq_stack now at the beginning

Now that the PDA is empty except for the stack canary, it can be removed.
The irqstack is moved to the start of the per-cpu section.  If the stack
protector is enabled, the canary overlaps the bottom 48 bytes of the irqstack.

tj: * updated subject
    * dropped asm relocation of irq_stack_ptr
    * updated comments a bit
    * rebased on top of stack canary changes
Signed-off-by: NBrian Gerst <brgerst@gmail.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Impact: cleanup

Make the following cleanups.

* remove duplicate comment from boot_init_stack_canary() which fits
  better in the other place - cpu_idle().

* move stack_canary offset check from __switch_to() to
  boot_init_stack_canary().
Signed-off-by: NTejun Heo <tj@kernel.org>