llvmPackages_13.libcxx: require gcc >=10 on gcc platforms

[risicle]

Motivation for this change

ZHF: #144627 @NixOS/nixos-release-managers

Specifically aarch64-linux is targeted here. As of version 13 libcxx does not build on gcc9: https://www.mail-archive.com/llvm-bugs@lists.llvm.org/msg53136.html

This is probably not the neatest way of doing this, mostly because it disobeys the stdenv that is passed in to the metapackage, instead sourcing it from pkgs in this case.

Most of the alternatives I could see though looked just about as bad. One possibility would be build all of llvmPackages_13 with gcc10, which could be done with a neater injection from all-packages.nix. But if a platform has stuck with gcc9, I'm assuming the issues with 10 are severe enough to not want to build an entire toolchain with it.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[7c6f434c]

Seems sensible.

[github-actions]

Successfully created backport PR #148460 for release-21.11.