x86/traps: Poison unused stack pointers in the TSS

x86/traps: Poison unused stack pointers in the TSS

This is for additional defence-in-depth following LDT/GDT/IDT corruption.

It causes attempted control transfers to ring 1 or 2 (via a call gate), or
attempts to use IST 3 through 7 to yield #SS, rather than executing with a
stack starting at the top of virtual address space.

Express the TSS setup in terms of structure assignment, which should be less
fragile if the IST indexes need to change, and has the useful side effect of
zeroing the reserved fields.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>