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/7/libunwind/default.nix

@@ -1,12 +1,12 @@
-{ lib, stdenv, version, fetch, fetchpatch, cmake, llvm
+{ lib, stdenv, llvm_meta, version, fetch, fetchpatch, cmake, llvm
 , enableShared ? !stdenv.hostPlatform.isStatic
 }:
 
-stdenv.mkDerivation {
+stdenv.mkDerivation rec {
   pname = "libunwind";
   inherit version;
 
-  src = fetch "libunwind" "035dsxs10nyiqd00q07yycvmkjl01yz4jdlrjvmch8klxg4pyjhp";
+  src = fetch pname "035dsxs10nyiqd00q07yycvmkjl01yz4jdlrjvmch8klxg4pyjhp";
 
   postUnpack = ''
     unpackFile ${llvm.src}
@@ -29,6 +29,8 @@ stdenv.mkDerivation {
     })
   ];
 
+  outputs = [ "out" "dev" ];
+
   nativeBuildInputs = [ cmake ];
 
   cmakeFlags = lib.optionals (!enableShared) [
@@ -36,4 +38,16 @@ stdenv.mkDerivation {
   ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     "-DLLVM_ENABLE_LIBCXX=ON"
   ];
+
+  meta = llvm_meta // {
+    # Details: https://github.com/llvm/llvm-project/blob/main/libunwind/docs/index.rst
+    homepage = "https://clang.llvm.org/docs/Toolchain.html#unwind-library";
+    description = "LLVM's unwinder library";
+    longDescription = ''
+      The unwind library provides a family of _Unwind_* functions implementing
+      the language-neutral stack unwinding portion of the Itanium C++ ABI (Level
+      I). It is a dependency of the C++ ABI library, and sometimes is a
+      dependency of other runtimes.
+    '';
+  };
 }