Do not depend on system tools on Darwin

[avdv]

Our cc wrapper was always using the system tools from a fixed location on Darwin, which is not hermetic (and can lead to problems when using nixpkgs) and requires the Xcode command line tools to be installed.

Apply the same heuristic as for the otool and install_name tool for the haskell toolchain and assume the tools are available at the same location as the ar binary.

Depends on https://github.com/tweag/rules_nixpkgs/pull/479 and depends on #2119

[avdv]

@mergifyio rebase

[mergify[bot]]

rebase

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