arch/x86: Support shadow stack on IRQ

For IRQs, shadow stack support a mechanism similar to the Interrupt
Stack Table (IST) for x86_64: a table, indexed by the IST index, pointing
to a 64 byte table in memory containing the address of seven shadow stacks
to be used by the interrupt service routines.

This patch adds support to this mechanism. It is worth noting that, as
Zephyr may exit from an interrupt by going to a different thread than
the one that was interrupted, some housekeeping is done to ensure that
the necessary shadow stack tokens are on the right shadow stack before
return from the interrupt.

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