x86: refactor mmustructs.h

The struct definitions for pdpt, pd, and pt entries has been
removed:

 - Bitfield ordering in a struct is implementation dependent,
   it can be right-to-left or left-to-right
 - The two different structures for page directory entries were
   not being used consistently, or when the type of the PDE
   was unknown
 - Anonymous structs/unions are GCC extensions

Instead these are now u64_t, with bitwise operations used to
get/set fields.

A new set of inline functions for fetcing various page table
structures has been implemented, replacing the older macros.

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