lib.systems: Simplify ABI on POWER for rust.rustcTarget

[OPNA2608]

Move the fix from just lib.systems.examples.ppc64-elfv1 into generic code, so it gets automatically applied for both ABI targets and any manually specified *System settings.

(If someone from the Rust team has any advice for how I could actually tell Rust what ABI it should use, or whether they even have such an option, please let me know! From browsing upstream issues & PRs, it's possible to query the ABI in use in Rust code, but I don't really see anything about telling the compiler which one it should actually use… Not urgent though, glibc ELFv2 is really low on my list of priorities.)

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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