haskell.compiler.ghc*: fall back to host libs for “cross native” ghc

The goal of this commit is basically to eliminate the use of
targetPackages for finding libraries. Instead, we introduce a
`targetLibs` set that can be used instead. The libraries in there
philosophically come from targetPackages since they are used by the core
libs and will be linked against user code. However, when cross compiling
GHC it's always a native compiler, so we can and have to use
pkgsHostTarget (targetPackages would be empty). This is explained more
in the acccompanying comment.

An alternative to this approach is not to pass in the libraries
explicitly via `--with-*` flags and rely on cc-wrapper and splicing to
pick the correct library. This works well for ncurses and probably
merits testing for other libraries as well since it's very simple. It
would need to be verified, however, that configure doesn't discover the
“wrong” library and leaks it somewhere.

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

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