x86: mmu: clarify physical/virtual conversions

The page table implementation requires conversion between virtual
and physical addresses when creating and walking page tables. Add
a phys_addr() and virt_addr() functions instead of hard-casting
these values, plus a macro for doing the same in ASM code.

Currently, all pages are identity mapped so VIRT_OFFSET = 0, but
this will now still work if they are not the same.

ASM language was also updated for 32-bit. Comments were left in
64-bit, as long mode semantics don't allow use of Z_X86_PHYS_ADDR
macro; this can be revisited later.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>