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/tools/build-managers/bazel/bazel_3/default.nix

@@ -371,7 +371,7 @@ stdenv.mkDerivation rec {
 
       # libcxx includes aren't added by libcxx hook
       # https://github.com/NixOS/nixpkgs/pull/41589
-      export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -isystem ${libcxx}/include/c++/v1"
+      export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -isystem ${lib.getDev libcxx}/include/c++/v1"
 
       # don't use system installed Xcode to run clang, use Nix clang instead
       sed -i -E "s;/usr/bin/xcrun (--sdk macosx )?clang;${stdenv.cc}/bin/clang $NIX_CFLAGS_COMPILE $(bazelLinkFlags) -framework CoreFoundation;g" \