Match up CFG architecture with Capstone architecture

[chrisseaton]

CFG files contain the architecture and word-size (just grep in them), but we currently assume they're always AMD64 and then hardcode this for Capstone.

Add a mapping from known CFG architectures and word-sizes (probably just containing AMD64 for now) to Capstone architectures and give an error if the CFG uses any architecture of word-size that we don't support.