x86_64: Missing a volatile on a struct used for spinning

Before we're initialized and can use proper synchronization, the CPU
initialization path spins on the thread entry function to be non-null.
But the data wasn't tagged volatile, and with gcc 8.2.1 (but not
6.2.0) the optimizer was hoisting the reads to SMP init would spin
forever.

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