arch/x86_64: Flag xuk shared page volatile

The shared page is inherently used in multiprocessor contexts where
the compiler optimizer can trip us up (specifically, a spin on
num_active_pus was being hoisted out of the loop on some gcc's).  Put
the volatile declartion into the struct pointer itself instead of
relying on the code to get it right.

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