licenses: convert license tags to SPDX

backport changes to ARM proofs from X64 work in progress

- replace ARM-specific constants and types with aliases which can be
instantiated separately for each architecture.
- expand lib with lemmas used in X64 proofs.
- simplify some proofs.

Also-by: Matthew Brecknell <Matthew.Brecknell@data61.csiro.au>

c-parser: support arch-specific testfiles