stdenv: promote zerocallusedregs to defaultHardeningFlags

[risicle]

Description of changes

Even when first introduced, this appeared to cause extremely few problems. I spotted an LLVM crash that appeared when using zerocallusedregs in building llvmPackages_17.libcxx but located the upstream issue (llvm/llvm-project#75168) and have applied a patch here. The fix is already present in LLVM 18.x.

Built hello on indicated platforms, also for pkgsStatic, pkgsMusl, pkgsCross.aarch64-multiplatform, pkgsi686Linux, pkgsLLVM variants.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

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[LeSuisse]

Should we update https://github.com/NixOS/nixpkgs/blob/master/doc/stdenv/stdenv.chapter.md#hardening-flags-enabled-by-default-sec-hardening-flags-enabled-by-default ?

[risicle]

Yes, good call - I'll also update the release notes..