arch/x86: (Intel64) implement SMP support

Add duplicate per-CPU data structures (x86_cpuboot, tss, stacks, etc.)
for up to 4 total CPUs, add code in locore and z_arch_start_cpu().

The test board, qemu_x86_long, now defaults to 2 CPUs.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>