x86: add support for common page tables

We provide an option for low-memory systems to use a single set
of page tables for all threads. This is only supported if
KPTI and SMP are disabled. This configuration saves a considerable
amount of RAM, especially if multiple memory domains are used,
at a cost of context switching overhead.

Some caching techniques are used to reduce the amount of context
switch updates; the page tables aren't updated if switching to
a supervisor thread, and the page table configuration of the last
user thread switched in is cached.

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