x86: gen_gdt: add address translation if needed

When the kernel is mapped into virtual address space
that is different than the physical address space,
the dynamic GDT generation uses the virtual addresses.
However, the GDT table is required at boot before
page table is loaded where the virtual addresses are
invalid. So make sure GDT generation is using
physical addresses.

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