x86: mmu: don't mark generic sections as present if desired
x86: mmu: don't mark generic sections as present if desired

With demand paging, it is possible for data pages to not be
present in physical memory. The gen_mmu.py script is updated
so that, if so desired, the generic sections are marked
non-present so the paging mechanism can bring them in
if needed.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>
finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr