llvmPackages_11.compiler-rt: restore `libcxxabi` argument

Removing this caused

`error: anonymous function at /var/lib/ofborg/checkout/repo/38dca4e3aa6bca43ea96d2fcc04e8229/mr-est/ofborg-evaluator-2/pkgs/development/compilers/llvm/11/compiler-rt/default.nix:1:1 called with unexpected argument 'libcxxabi'`

https://github.com/search?q=repo%3ANixOS%2Fnixpkgs%20%2Fcompiler-rt.override%2F&type=code

The argument became unused in https://github.com/nixos/nixpkgs/commit/56fcbcd1543188fcc855df7e79fee0a8df259681#diff-b1bb77941db4393b7d2cf7a370cee294318418f2507b7a78e22353d1afbb6d21L18

but it was not removed from `buildInputs` in later `llvmPackages` because `llvmPackages_12` had
been created before this change was done.

Removing `libcxxabi` from `buildInputs` may have to be applied to the
other `llvmPackages` too.