boards: qemu_x86/atom/virt: forcibly disable memory mapped stack

The current memory mapped stack code requires actual physical
addresses for stacks, and cannot deal with stacks already using
virtual addresses. So disable mapped stack via defconfig.

Note that this is done before enabling memory mapped stacks on
x86 so test won't fail when the support in the architecture
code is introduced.

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