llvmPackages_11.compiler-rt: fix build on darwin

[r-burns]

Needed to prevent faulty i686 codegen on x86_64. We're doing this
already for the older llvm packages, so not sure why 11 never had this.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[SuperSandro2000]

This is a semi-automatic executed nixpkgs-review with nixpkgs-review-checks extension. It is checked by a human on a best effort basis and does not build all packages (e.g. lumo, tensorflow or pytorch).
If you have any questions or problems please reach out to SuperSandro2000 on IRC.

Result of nixpkgs-review pr 111701 run on x86_64-darwin 1

2 packages marked as broken and skipped:

• puppeteer-cli
• zig

2 packages built:

• clang-analyzer
• clang_11

[Luflosi]

LGTM.