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:

binexport/ida/names.cc

Line 87 in 7510f51

switch (GetArchitecture()) {

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.