Add a flag to export symbols for call return adresses.

Motivation

When looking at ghc backtraces return points of calls often show up as things like <ghc_MkIface_mkIface2_slow+76416>

This was changed in https://phabricator.haskell.org/D4722 which in general I agree with.

However when debugging low level issues I will take any help I can get and having these symbols available would be quite helpful.

Add a flag to be able to revert this behavior for debugging purposes.

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