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/interpreters/pure/default.nix

@@ -1,5 +1,5 @@
 { lib, stdenv, fetchurl, makeWrapper,
-  llvm, gmp, mpfr, readline, bison, flex }:
+  libllvm, gmp, mpfr, readline, bison, flex }:
 
 stdenv.mkDerivation rec {
   baseName="pure";
@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ makeWrapper ];
   buildInputs = [ bison flex ];
-  propagatedBuildInputs = [ llvm gmp mpfr readline ];
+  propagatedBuildInputs = [ libllvm gmp mpfr readline ];
   NIX_LDFLAGS = "-lLLVMJIT";
 
   postPatch = ''
@@ -25,10 +25,10 @@ stdenv.mkDerivation rec {
   configureFlags = [ "--enable-release" ];
   doCheck = true;
   checkPhase = ''
-    LD_LIBRARY_PATH=$LD_LIBRARY_PATH''${LD_LIBRARY_PATH:+:}${llvm}/lib make check
+    LD_LIBRARY_PATH=$LD_LIBRARY_PATH''${LD_LIBRARY_PATH:+:}${libllvm}/lib make check
   '';
   postInstall = ''
-    wrapProgram $out/bin/pure --prefix LD_LIBRARY_PATH : ${llvm}/lib
+    wrapProgram $out/bin/pure --prefix LD_LIBRARY_PATH : ${libllvm}/lib
   '';
 
   meta = {