Compiler ABI hash should include the toolchain

[mpickering]

Currently cabal-install will create one store directory per distinct compiler. However this is not quite correct since it only consults the "Project Unit Id" field, which records a hash of the build configuration which produced the compiler, but doesn't include toolchain information.

For example, if you install the same compiler twice with two different toolchains then cabal-install will reuse the same store directory, however, this is very likely to not work correctly. For instance, you can't typically use clang to link together object files produced by gcc or vice-versa.

Therefore it would be better to include more information in the hash. The settings file seems appropriate, as that contains (1) The Project Unit Id, which records information about the build, and also (2) any information about the configured toolchain that is discovered during installation.

Related to #10170