llvm8: support c++ in cross case

this adds libc++ to the LLVM cross, giving us access to the full
Nixpkgs set. This requires 4 stages of wrapped compilers:

- Clang with no libraries
- Clang with just compiler-rt
- Clang with Libc, and compiler-rt
- Clang with Libc++, Libc, and compiler-rt

pkgs/development/libraries/kerberos/krb5.nix

@@ -39,8 +39,9 @@ stdenv.mkDerivation rec {
     ++ optional (!libOnly) yacc
     # Provides the mig command used by the build scripts
     ++ optional stdenv.isDarwin bootstrap_cmds;
+
   buildInputs = [ openssl ]
-    ++ optionals (stdenv.hostPlatform.isLinux && stdenv.hostPlatform.libc != "bionic") [ keyutils ]
+    ++ optionals (stdenv.hostPlatform.isLinux && stdenv.hostPlatform.libc != "bionic" && !(stdenv.hostPlatform.useLLVM or false)) [ keyutils ]
     ++ optionals (!libOnly) [ openldap libedit ];
 
   preConfigure = "cd ./src";