ar flags are too rigid

**Describe the bug**
Currently Cabal hard-codes the flags passed to `ar` on a per-platform basis. This went wrong in [GHC #23188](https://gitlab.haskell.org/ghc/ghc/-/issues/23188), where using `llvm-ar` on OS X lead to the production of an invalid static archive. In this particular case, `Cabal` failed to use the `ar L` flag, resulting in a nested archive.

**To Reproduce**
On a Darwin box:

```
brew install llvm@11
curl https://downloads.haskell.org/ghc/9.6.1/ghc-9.6.1-aarch64-apple-darwin.tar.bz2 | tar -x
cd ghc-9.6.1-aarch64-apple-darwin/
export AR=/opt/homebrew/opt/llvm@11/bin/llvm-ar
export CC=/opt/homebrew/opt/llvm@11/bin/clang
export CXX=/opt/homebrew/opt/llvm@11/bin/clang++
export RANLIB=/opt/homebrew/opt/llvm@11/bin/llvm-ranlib
./configure --prefix=`pwd`/root
make install

git clone https://github.com/haskell/haskell-language-server
cd haskell-language-server
cabal build -w $HOME/ghc-9.6.1-aarch64-apple-darwin/root/bin/ghc
```

**Expected behavior**
Cabal use appropriate flags for the platform's `ar` implementation.

**System information**
- Operating system
- `cabal`, `ghc` versions

**Additional context**
Add any other context about the problem here.

Reproduce the failure on Darwin with the llvm-ar, clang, and GHC setup shown in the issue, then trace Cabal's platform-specific handling of ar flags. Done means Cabal selects flags appropriate to the ar implementation and the demonstrated haskell-language-server build succeeds without producing an invalid or nested archive.

Written by the indexing model from the issue text.