soc/x86_64: board/x86_64/qemu_x86_64: Enable 2-processor SMP as default

We can turn this off in application code, but the default should be
SMP.

Oddball note: the SMP setting is in the soc layer and not arch, even
though I think it would be better there.  We don't actually have a
per-arch defconfig file included by kconfig.

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