llvm 3.8+: Remove unneeded libcxxabi dependency

Evidentally this hasn't been needed for a while

pkgs/development/compilers/llvm/6/default.nix

@@ -25,9 +25,7 @@ let
     callPackage = newScope (tools // { inherit stdenv cmake libxml2 python2 isl release_version version fetch; });
   in {
 
-    llvm = overrideManOutput (callPackage ./llvm.nix {
-      inherit (targetLlvmLibraries) libcxxabi;
-    });
+    llvm = overrideManOutput (callPackage ./llvm.nix { });
     clang-unwrapped = overrideManOutput (callPackage ./clang {
       inherit clang-tools-extra_src;
     });

pkgs/development/compilers/llvm/6/llvm.nix

@@ -11,7 +11,6 @@
 , version
 , release_version
 , zlib
-, libcxxabi
 , debugVersion ? false
 , enableManpages ? false
 , enableSharedLibraries ? true
@@ -40,8 +39,7 @@ in stdenv.mkDerivation (rec {
   nativeBuildInputs = [ cmake python ]
     ++ stdenv.lib.optional enableManpages python.pkgs.sphinx;
 
-  buildInputs = [ libxml2 libffi ]
-    ++ stdenv.lib.optionals stdenv.isDarwin [ libcxxabi ];
+  buildInputs = [ libxml2 libffi ];
 
   propagatedBuildInputs = [ ncurses zlib ];