x86: gen_mmu: double map physical/virtual memory at top level

This reuses the page directory pointer table (PAE=y) or page
directory (PAE=n) to point to next level page directory table
(PAE=y) or page tables (PAE=n) to identity map the physical
memory. This gets rid of the extra memory needed to host
the extra mappings which are only used at boot. Following
patches will have code to actual unmap physical memory
during the boot process, so this avoids some wasting of
memory.

Since no extra memory needs to be reserved, this also reverts
commit ee3d345c
("x86: mmu: reserve more space for page table if linking in virt").

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