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/12/lld/default.nix

@@ -18,6 +18,13 @@ stdenv.mkDerivation rec {
     ./gnu-install-dirs.patch
   ];
 
+  postPatch = ''
+    substituteInPlace MachO/CMakeLists.txt --replace \
+      '(''${LLVM_MAIN_SRC_DIR}/' '('
+    mkdir -p libunwind/include
+    tar -xf "${libunwind.src}" --wildcards -C libunwind/include --strip-components=2 "libunwind-*/include/"
+  '';
+
   nativeBuildInputs = [ cmake ];
   buildInputs = [ libllvm libxml2 ];
 
@@ -29,13 +36,6 @@ stdenv.mkDerivation rec {
 
   outputs = [ "out" "lib" "dev" ];
 
-  postPatch = ''
-    substituteInPlace MachO/CMakeLists.txt --replace \
-      '(''${LLVM_MAIN_SRC_DIR}/' '('
-    mkdir -p libunwind/include
-    tar -xf "${libunwind.src}" --wildcards -C libunwind/include --strip-components=2 "libunwind-*/include/"
-  '';
-
   meta = llvm_meta // {
     homepage = "https://lld.llvm.org/";
     description = "The LLVM linker";
@@ -43,7 +43,7 @@ stdenv.mkDerivation rec {
       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
+      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.
     '';