llvmPackages: Fix more inconsistencies

The main thing was using `llvm_meta` in all versions.

Secondarily:

 - libunwindx7: Forgot to split outputs

 - libcxx{,abi} 12: Forgot to apply output-splitting patches.

 - simplify `useLLVM` stdenv-switching logic.

 - openmp always gets its own directory

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

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetch, cmake, python3, libcxxabi, fixDarwinDylibNames, version }:
+{ lib, stdenv, llvm_meta, fetch, cmake, python3, libcxxabi, fixDarwinDylibNames, version }:
 
 stdenv.mkDerivation {
   pname = "libcxx";
@@ -45,10 +45,15 @@ stdenv.mkDerivation {
     isLLVM = true;
   };
 
-  meta = {
+  meta = llvm_meta // {
     homepage = "https://libcxx.llvm.org/";
-    description = "A new implementation of the C++ standard library, targeting C++11";
-    license = with lib.licenses; [ ncsa mit ];
-    platforms = lib.platforms.unix;
+    description = "C++ standard library";
+    longDescription = ''
+      libc++ is an implementation of the C++ standard library, targeting C++11,
+      C++14 and above.
+    '';
+    # "All of the code in libc++ is dual licensed under the MIT license and the
+    # UIUC License (a BSD-like license)":
+    license = with lib.licenses; [ mit ncsa ];
   };
 }