fix(Cabal): Use the C toolchain GHC was configured with

[zlonast]

fix: #12332

Cabal now asks GHC what toolchain it was configured with (via ghc --info,
i.e. the settings file chosen when GHC or its bindist was configured)

  $ ghc --info | grep -E '"C compiler command"|"Merge objects command"'
  ,("C compiler command","/usr/bin/clang")
  ,("Merge objects command","/usr/bin/ld.lld")
  $ cabal build -v2 --enable-library-for-ghci
  Running: /usr/bin/ar -r dist/build/libHSpkg-0.1-inplace.a '@dist/tmp/ar.rsp'
- Running: /usr/bin/ld --help
- Running: /usr/bin/ld -x -r /tmp/cabal-1f3a5-5.o -o /tmp/cabal-1f3a5-6.o
- Running: /usr/bin/ld -r -o dist/build/libHSpkg-0.1-inplace.o '@dist/tmp/ld.rsp'
+ Running: /usr/bin/ld.lld --help
+ Running: /usr/bin/ld.lld -x -r /tmp/cabal-1f3a5-5.o -o /tmp/cabal-1f3a5-6.o
+ Running: /usr/bin/ld.lld -r -o dist/build/libHSpkg-0.1-inplace.o '@dist/tmp/ld.rsp'

---

Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)