hsc2hs shouldn't call cc directly

Currently `hsc2hs` calls `cc` directly when it needs to compile C. However, in general it doesn't have enough information about the build context to do so robustly, especially when GHC becomes runtime-retargetable (since the choice of which C compiler is used is going to depend upon the GHC profile). This is already starting to affect us as `hsc2hs` isn't benefitting from upstream GHC changes like https://gitlab.haskell.org/ghc/ghc/-/issues/20162.

I propose that hsc2hs should consistently rely on `ghc` to invoke the C compiler, centralizing build configuration in GHC.

Start by tracing hsc2hs's path for compiling C and identify where it invokes cc directly; the issue does not name files or tests. Compare that path with how ghc invokes the C compiler, then verify that hsc2hs consistently delegates through ghc so build configuration and compiler selection are centralized.

Written by the indexing model from the issue text.