[WIP] Cross compilation changes, with enphasis on (haskell) GHC HEAD

[Ericson2314]

Motivation for this change

This is initially a dump of a bunch of fixes and improvement I did for cross compilation for work---quite accelerating the pace I originally had in mind to do on my own :). I did slap-dash merge to put something up here (which seems to already have a conflict...) mainly to get a view-able diff up for those asking me about this. Within the next 2 weeks I'll try to make more progress cleaning things up so this is in a reviewable and mergeable state---the prior work which makes this cleanable is now done as of today so time to get the ball rolling.

This doesn't quite subsume #21268 as that as some further hash-preserving cleanups (e.g. the "build wrappers" file), which aren't part of this. Once this is in, that should be easy to rebase.

I doubt there is a good way to farm out cleaning up git history, but if anyone want to give it a shot, I'd really, really appreciate.

CC @dmjio @bgamari @elitak @copumpkin

Things done

• Leverage Platform normalization #24610 (e.g. no more targetPlatform.config or "" needed).
• Squash history into something reviewable.
• Remove unneeded GHC patches because stuff has been upstreamed in that project.

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[mention-bot]

@Ericson2314, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @bgamari, @bjornfor, @LnL7 and @copumpkin to be potential reviewers.

[elitak]

I'm not in charge of merges, but I think this is too much in one go. I suggest cherry-picking the "fixed cross compilation" and version bump commits into a separate PR, then rebasing this branch onto that one, then finding parts of the remaining changes that touch packages outside of ghc and separating them likewise.

Also extricate the changes to lib/ and stdenv/ into separate PRs, keeping the non-breaking changes from the ones that should have no impact on master.

[Ericson2314]

@elitak indeed it is too big -- I mean this more to be a progress log as I rebase than something to actually merge.

#25047 is my first attempt to break off a bit and clean it up. Do you think that is small enough?

[Ericson2314]

Closing cause I finally cleaned up the history :D, but better to use a new PR.