Create a registry of architecture/ISA names

[cblichmann]

Historically, we have avoided using an enum for architecture names, as we wanted to stay flexible. However, to aid tooling, there should be a definitive list of architecture/ISA names to be used in BinExport metadata.

The IDA Pro plugin has this and can be used as a start: https://github.com/google/binexport/blob/7510f51ce717b69b59ca72a4a1c9d357b4ef932f/ida/names.cc#L87

This could be implemented simply by documenting architectures in the repo somewhere, but some helper code to verify and/or extract relevant bits would be useful.

For a future, incompatible format, BinExport could just reuse LLVM's target triple.