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/9/lld/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv
+{ lib, stdenv, llvm_meta
 , buildLlvmTools
 , fetch
 , cmake
@@ -28,10 +28,16 @@ stdenv.mkDerivation rec {
 
   outputs = [ "out" "lib" "dev" ];
 
-  meta = {
-    description = "The LLVM Linker";
-    homepage    = "https://lld.llvm.org/";
-    license     = lib.licenses.ncsa;
-    platforms   = lib.platforms.all;
+  meta = llvm_meta // {
+    homepage = "https://lld.llvm.org/";
+    description = "The LLVM linker";
+    longDescription = ''
+      LLD is a linker from the LLVM project that is a drop-in replacement for
+      system linkers and runs much faster than them. It also provides features
+      that are useful for toolchain developers.
+      The linker supports ELF (Unix), PE/COFF (Windows), Mach-O (macOS), and
+      WebAssembly in descending order of completeness. Internally, LLD consists
+      of several different linkers.
+    '';
   };
 }