Should Cabal build foreign source files in parallel as well?

Currently, Cabal does not build foreign source files (C++/C/ASM/CMM) in a parallel manner when building with `-j`. This can be a problem when we try to build a Haskell package which bundles a non-trivial C/C++ project. Would Cabal folks review a PR which adds parallel building for foreign sources, or is the extra complexity not worth the benefits?

The issue names no files, tests, or entry points. Start by locating Cabal's build scheduling logic for foreign C, C++, ASM, and CMM sources, then determine the dependency and ordering requirements; done means a reviewed design and tests demonstrating safe parallel foreign-source builds under -j.

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