Avoid linking of custom Setup scripts

**Describe the bug**
Currently custom `Setup.hs` scripts are compiled and linked into an executable, which is then executed. On many platforms (e.g. Windows) this can be a considerable fraction of the overall package build time due to the high cost of linking.

Ideally we would rather use the bytecode interpreter to execute such one-off executables. I am aware of two reasons why this hasn't been done in the past:

* the bytecode interpreter has historically supported only a subset of the Haskell language (e.g. lacking support for unboxed tuples).
* the only convenient way to run a script using the bytecode interpreter is via `runghc`, which `Cabal` has no way of reliably locating

With the 9.2 release, GHC's bytecode interpreter will support all Haskell constructs, eliminating the first of these considerations. To eliminate the second I [propose](https://gitlab.haskell.org/ghc/ghc/-/issues/18011) that we introduce a new `--run` mode to GHC. This would be similar to the existing `--eval` mode, but specialised the use-case of executing a script. For instance, `Cabal` might run run:

```
ghc --run Setup.hs -- configure
```

Thoughts?

Start by tracing how Cabal compiles, links, and executes a custom Setup.hs script, then review the proposed GHC --run mode and the existing runghc and --eval approaches. Done should avoid linking one-off Setup scripts while preserving configure and other Setup.hs command behavior across supported platforms.

Written by the indexing model from the issue text.