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.

Edited Jan 31, 2024 by Rodrigo Mesquita

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