aspec/haskell/machine: refactor user_context interface

- remove separate abstract set_/get_register implementation, directly use machine op
- make interface aware that user_context does not always need to equal
(register => machine_word)
- introduce FPU state on x64

fix ARM build after merge

Also:
- move some ARM-specific things out of Tcb_AI
- port changes from ARM to X64, up to beginning of ArchVSpace_AI

x64: port device-untyped from ARM

x64: add arch_split'd x64 spec with IOMMU stuff