For smp we need a way to maintain thread local state. The bottom
of the thread stack is a good place, and is where Linux puts it.
So we just steal the concept of the thread_info structure that
lives at the bottom of the stack in Linux, and introduce it to
kvm-unit-tests/arm[64]. For starters we just have cpu index for
state, and that's implicitly initialized to zero for CPU0 already.
So, as we don't have secondary cpus yet, there's not much to do.

Additionally, sneak a small fixup in to the initial stack setup
for arm64. We were assuming that spsel is EL1 after reset, which
has been true so far, but let's not assume.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>