llvmPackages_13.compiler-rt: Mark as broken on Aarch64

[primeos]

To avoid unnecessary builds but this needs to be fixed ASAP. Chromium
already depends on it and a lot of additional packages, including Mesa,
will depend on it after the stable release.

Motivation for this change

I made some progress in #136574 but it needs more work and I lack time.

cc @thefloweringash as the Aarch64 maintainer for Chromium

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 via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution 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
• Fits CONTRIBUTING.md.

[github-actions]

Successfully created backport PR #136640 for release-21.05.

[github-actions]

The process '/usr/bin/git' failed with exit code 1

[r-burns]

How about just disabling libfuzzer?

[primeos]

@r-burns unfortunately I don't have time to take a look, as I noted above. Feel free to submit a PR.