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/intel-graphics-compiler/default.nix

@@ -18,7 +18,7 @@ let
     inherit spirv-llvm-translator;
   };
   inherit (llvmPkgs) llvm;
-  inherit (if buildWithPatches then opencl-clang else llvmPkgs) clang clang-unwrapped spirv-llvm-translator;
+  inherit (if buildWithPatches then opencl-clang else llvmPkgs) clang libclang spirv-llvm-translator;
   inherit (lib) getVersion optional optionals versionOlder versions;
 in
 
@@ -55,7 +55,7 @@ stdenv.mkDerivation rec {
       ln -s ${clang}/bin/clang $out/
       ln -s clang $out/clang-${versions.major (getVersion clang)}
       ln -s ${opencl-clang}/lib/* $out/
-      ln -s ${clang-unwrapped}/lib/clang/${getVersion clang}/include/opencl-c.h $out/
+      ln -s ${lib.getLib libclang}/lib/clang/${getVersion clang}/include/opencl-c.h $out/
     '';
   };