cc-wrapper: add support for spectrev2 hardening flag

[risicle]

Description of changes

This enables the "retpoline" protection on supported compilers for hardening against spectre v2 ("Branch Target Injection") attacks.

There are comparatively few packages that are likely to be vulnerable to this kind of attack, and packages like the kernel handle these flags themselves - so this flag is mostly for the paranoid or users of "hardened" systems (what's the difference har har). Though it is a good demonstrator of how to implement platform-specific hardening flags via hardeningUnsupportedFlagsByTargetPlatform.

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/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 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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