kernel: interrupt/idle stacks/threads as array

The set of interrupt stacks is now expressed as an array. We
also define the idle threads and their associated stacks this
way. This allows for iteration in cases where we have multiple
CPUs.

There is now a centralized declaration in kernel_internal.h.

On uniprocessor systems, z_interrupt_stacks has one element
and can be used in the same way as _interrupt_stack.

The IRQ stack for CPU 0 is now set in init.c instead of in
arch code.

The extern definition of the main thread stack is now removed,
this doesn't need to be in a header.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>