Add gcc path

[zlonast]

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

show-build-info with -pgmc gives new output on each platform #10969 (comment)

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[ulysses4ever]

Can you please write a quick (but legible) self-contained explanation of why we want this?

[zlonast]

@ulysses4ever philosophically I'm trying to convince myself to write a few dozen link tests 😅

the real problem is that when adding -pgm* flags we need to write at least 5(10) tests instead of one because the output contains the path to the C compiler

example from the pull request I'm referring to:

1. no -pgmc
single-2.out
single-2.test.hs
2. gcc is /usr/bin/gcc
single.out
single.test.hs
3. clang is /tools/ghc-<GHCVER>/lib/../mingw/bin/clang
win-single-4.out
win-single-4.test.hs
4. clang is /ghcup/ghc/<GHCVER>/lib/../mingw/bin/clang
win-single-8-6.out
win-single-8-6.test.hs
5. clang is /ghcup/ghc/<GHCVER>/lib/../mingw//bin/clang
win-single.out
win-single.test.hs

[geekosaur]

That sounds more like a bug / deficiency in the normalizer to me.

[zlonast]

That sounds more like a bug / deficiency in the normalizer to me.

What is a normalizer?

[geekosaur]

It's a postprocessor for test output that produces those <UPPERCASE> things in place of mutable paths.
https://github.com/haskell/cabal/blob/master/cabal-testsuite/src/Test/Cabal/OutputNormalizer.hs