Replace the removed initialization within setup_percpu.c with the
initialization for X86_64 defined within irq_64.c
Change the description accordingly.
Replace irq_stack_union with fixed_percpu_data
Update to the current system map
Update description of initialization process
Replace DECLARE macros with the current implementation
Within /arch/x86/kernel/head_64.S the implementation of the
initialization was changed.
Update the passage accordingly.
b1bd27b9ad (diff-a136f03867893e5d01eeadaba59c2dff)
Also fix a typo from a previous commit.
The irq_stack is no longer within a irq_stack_union
but separated into the irq_stack struct and the fixed_percpu_data struct
This change was made with the following series of commits:
e6401c1309 (diff-7db868ab08485b2578c9f97e45fb7d00)
