[kernel] Remove #if WITH_DEV_IOMMU_* conditionals

WITH_DEV_IOMMU_DUMMY is always defined.
Replace WITH_DEV_IOMMU_INTEL with ARCH_X86.

Test: CQ & manual verification that kernel text size didn't change
Change-Id: I4e31c854a88da6d116e65f65494b92572a2c3956

[syscalls] Update parameter types and names

This CL updates the parameter types and names for a number of syscalls
to match the Zircon System API rubric. There's more work to do to make
all the syscalls conform to the rubric, but this change is a start.

Change-Id: I218ac5e7e0cbd80a8c69fef7891ad40b78b6b407

[iommu][intel] Beginnings of implementation

This provides a basic implementation of VT-d. It has the following
known limitations:

- No support for first level translation or requests-with-PASID
- Only supports 48-bit second level address spaces
- Currently performs identity-mapping only (i.e. acts more as a MPU
rather than an MMU)
- Performs synchronous IOTLB invalidations rather than using the
queued-invalidation feature.
- Does not support handing off across mexec

ZX-693 #comment

Change-Id: I6b2a7e49c5064065c2b6d06787bbd467fd666119

[object][iommu] Wire DummyIommu into IommuDispatcher

ZX-693

Change-Id: I54e970b299d646c80712c92ec3f702ec774b0f0d

[object][iommu] Implement the IommuDispatcher

ZX-693

Change-Id: I30d91eae7a96ee88c0f865b1e861d3ceb5da47f1