arch/x86: restructure ISR stacks (conceptually)

This is largely a conceptual change rather than an actual change.
Instead of using an array of interrupt stacks (one for each IRQ
nesting level), we use one interrupt stack and subdivide it. The
effect is the same, but this is more in line with the Zephyr model
of one ISR stack per CPU (as reflected in init.c).

Signed-off-by: Charles E. Youse <charles.youse@intel.com>