ignore `llvmPackages_latest`, it's an alias and it'll be a duplicate.
ignore `llvmPackages_git`, it's not supposed to be used in nixpkgs and
may be broken.

Show what libc is used, otherwise there's indication what libc is being
used. Ex in gccMultiStdenv.

this is much easier to read.

…g for linux and x86_64

they're only supported on x86_64-linux

This reverts commit d33a309.

commit caused ofborg-eval to fail with

```
$ nix-build pkgs/top-level/release.nix -A tests.cc-wrapper.gcc10Stdenv.aarch64-darwin
       error: evaluation aborted with the following error message: 'cannot find attribute `tests.cc-wrapper.gcc10Stdenv''
```

on the 3 other platfomrs the attr is there

filter out llvmPackages <=10 on aarch64-darwin, they're marked broken
and availableOn doesn't filter broken packages