ghc-toolchain: Backwards compatibility for target descriptions
I realize now that we don't have a compatibility story for target description files yet. Scenario:
- Configure a toolchain with ghc-toolchain 0.1
- Update compiler/ghc-toolchain
- Read an existing toolchain using a program linked against ghc-toolchain 0.2, which has additional fields.
- The existing toolchain will fail to parse.
We already read from a serialized version of the datatype, so I suppose we could either assume some defaults, or ask to reconfigure.