x86: gen_mmu: add address translation if needed

There is an assumption made in the page table generation code
that the kernel would occupy the same physical and virtual
addresses. However, we may want to map the kernel into
a virtual address space which differs from kernel's physical
address space. For example, with demand paging enabled on
kernel code and data, we can accommodate kernel that is
larger than physical memory size, and may want to utilize
a bigger virtual address space. So add address translation
in the gen_mmu.py script for this.

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