[x86][mmu] Overhaul TLB invalidation

We previously did TLB shootdowns one mapping at a time, potentially
generating many IPIs per arch_mmu_* operation. This change coalesces
the shootdown into a single IPI per arch_mmu_* operation. Additionally,
if many pages are being invalidated, we now will perform a full flush
rather than an arbitrary number of single page invalidations.

ZX-979 #done

Change-Id: I49cb9a6759d1a1a86e1cfd18e3364fc6c0157687

[x86][mmu] Move the MMU code into a library

In the near future, there will be a user of this that is not the
logical CPU (VT-d).

Change-Id: I115ab92fb5730ac4575298e66f0d622cb8b4821e