x86: x86_64 can only support max 4 CPUs

With all the stacks and TSS (etc), the x86_64 arch code can only
support maximum of 4 CPUs at the moment. So add a build assert
if more CPUs are specified via CONFIG_MP_MAX_NUM_CPUS, also
overwrite the range value for CONFIG_MP_MAX_NUM_CPUS.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>