llvmPackages: Multuple outputs for everythting

Also begin to start work on cross compilation, though that will have to
be finished later.

The patches are based on the first version of
https://reviews.llvm.org/D99484. It's very annoying to do the
back-porting but the review has uncovered nothing super major so I'm
fine sticking with what I've got.

Beyond making the outputs work, I also strove to re-sync the packages,
as they have been drifting pointlessly apart for some time.

----

Other misc notes, highly incomplete

- lvm-config-native and llvm-config are put in `dev` because they are
  tools just for build time.

- Clang no longer has an lld dep. That was introduced in
  db29857eb3, but if clang needs help
  finding lld when it is used we should just pass it flags / put in the
  resource dir. Providing it at build time increases critical path
  length for no good reason.

----

A note on `nativeCC`:

`stdenv` takes tools from the previous stage, so:

1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.stdenv.cc`: `(?0, ?1, x)`

while:

1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.targetPackages`: `(x, x, ?2)`
3. `pkgsBuildBuild.targetPackages.stdenv.cc`: `(?1, x, x)`

pkgs/development/compilers/llvm/12/lld/default.nix

@@ -1,9 +1,10 @@
 { lib, stdenv, llvm_meta
+, buildLlvmTools
 , fetch
 , libunwind
 , cmake
 , libxml2
-, llvm
+, libllvm
 , version
 }:
 
@@ -13,8 +14,20 @@ stdenv.mkDerivation rec {
 
   src = fetch pname "1zakyxk5bwnh7jarckcd4rbmzi58jgn2dbah5j5cwcyfyfbx9drc";
 
+  patches = [
+    ./gnu-install-dirs.patch
+  ];
+
   nativeBuildInputs = [ cmake ];
-  buildInputs = [ llvm libxml2 ];
+  buildInputs = [ libllvm libxml2 ];
+
+  cmakeFlags = [
+    "-DLLVM_CONFIG_PATH=${libllvm.dev}/bin/llvm-config${lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) "-native"}"
+  ] ++ lib.optionals (stdenv.hostPlatform != stdenv.buildPlatform) [
+    "-DLLVM_TABLEGEN_EXE=${buildLlvmTools.llvm}/bin/llvm-tblgen"
+  ];
+
+  outputs = [ "out" "lib" "dev" ];
 
   postPatch = ''
     substituteInPlace MachO/CMakeLists.txt --replace \
@@ -23,13 +36,6 @@ stdenv.mkDerivation rec {
     tar -xf "${libunwind.src}" --wildcards -C libunwind/include --strip-components=2 "libunwind-*/include/"
   '';
 
-  outputs = [ "out" "dev" ];
-
-  postInstall = ''
-    moveToOutput include "$dev"
-    moveToOutput lib "$dev"
-  '';
-
   meta = llvm_meta // {
     homepage = "https://lld.llvm.org/";
     description = "The LLVM linker";