arch/x86_64: Add a per-CPU SMP bringup API

The previous scheme where the xuk layer would call out to the to
"fetch" the stack for a SMP CPU at startup was sorta weird, and an
impedance mismatch with Zephyr which has a "start this CPU" call
instead.  It also got broken when x86_64 started launching CPUs
(correctly) on their interrupt stacks instead of a temporary area;
they weren't ready yet when xuk initialization was happening and the
system would deadlock waiting for code that can't run yet to provide a
stack.

Note that this preserves the somewhat quirky behavior that Zephyr's
CPU numbering is just the order in which the SMP CPUs emerge from
initialization and not a hardware ID.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>