Loader checks to ensure plugin soundness with reinstallable GHC

@wz1000 noted an interesting difficulty when working on reinstallable GHC: plugins require a fair amount of care. In particular, imagine that we have the following:

Here we see the build graph of a typical GHC build in a world where the ghc library is reinstallable:

• Hadrian uses the bootstrap (stage 0) compiler to build a stage 1 ghc and ghc-bin
• Hadrian uses the stage 1 compiler to build ghc-prim, base, template-haskell, etc.
• Hadrian uses the stage 1 compiler to build stage 2 ghc and ghc-bin
• the user (e.g. using cabal-install) uses the stage 2 ghc-bin to build a "stage 2b" ghc and ghc-bin
• the user wants to use the stage 2 toolchain to build a package using a compiler plugin

This situation requires great care. In particular, we need to ensure that the plugin is linked against the same ghc library that the ghc-bin it is loaded into is linked against. The linker needs to ensure that this is the case and raise an error when it's not.

Currently this is problematic as ghc.cabal overrides its unit ID, meaning that the stage 2 and stage 2b ghc libraries will be indistinguishable. This will need to change.