arch/x86: Support shadow stack on nested IRQs

Nested interrupts are supported, on the normal stack, by creating a
stack whose size is a multiple of CONFIG_ISR_DEPTH, and updating the
pointer used by Interrupt Stack Table (IST) to point to a new base,
inside the "oversized" stack.

The same approach is used for the shadow stack: shadow stack size is
multiplied by CONFIG_ISR_DEPTH, and the pointer to the stack on the
shadow stack pointer table is update to point to the next base.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>