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/rocm/default.nix

@@ -1,4 +1,4 @@
-{ lib, fetchFromGitHub, callPackage, wrapCCWith }:
+{ lib, buildPackages, fetchFromGitHub, callPackage, wrapCCWith }:
 
 let
   version = "4.1.0";
@@ -15,7 +15,7 @@ in rec {
       clang_version=`${cc}/bin/clang -v 2>&1 | grep "clang version " | grep -E -o "[0-9.-]+"`
       rsrc="$out/resource-root"
       mkdir "$rsrc"
-      ln -s "${cc}/lib/clang/$clang_version/include" "$rsrc"
+      ln -s "${lib.getLib cc}/lib/clang/$clang_version/include" "$rsrc"
       echo "-resource-dir=$rsrc" >> $out/nix-support/cc-cflags
       echo "-Wno-unused-command-line-argument" >> $out/nix-support/cc-cflags
       rm $out/nix-support/add-hardening.sh
@@ -31,6 +31,7 @@ in rec {
   lld = callPackage ./lld {
     inherit llvm version;
     src = "${src}/lld";
+    buildLlvmTools = buildPackages.llvmPackages_rocm;
   };
 
   llvm = callPackage ./llvm {