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/python-modules/kiwisolver/1_1.nix

@@ -14,7 +14,7 @@ buildPythonPackage rec {
     sha256 = "53eaed412477c836e1b9522c19858a8557d6e595077830146182225613b11a75";
   };
 
-  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${libcxx}/include/c++/v1";
+  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${lib.getDev libcxx}/include/c++/v1";
 
   # Does not include tests
   doCheck = false;

pkgs/development/python-modules/kiwisolver/default.nix

@@ -15,7 +15,7 @@ buildPythonPackage rec {
     sha256 = "950a199911a8d94683a6b10321f9345d5a3a8433ec58b217ace979e18f16e248";
   };
 
-  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${libcxx}/include/c++/v1";
+  NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin "-I${lib.getDev libcxx}/include/c++/v1";
 
   nativeBuildInputs = [
     cppy