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/7/libunwind/default.nix

@@ -8,7 +8,9 @@ stdenv.mkDerivation {
 
   src = fetch "libunwind" "035dsxs10nyiqd00q07yycvmkjl01yz4jdlrjvmch8klxg4pyjhp";
 
-  patches = lib.optional (stdenv.hostPlatform.useLLVM or false) [
+  patches = [
+    ./gnu-install-dirs.patch
+  ] ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
     # removes use of `new` that require libc++
     (fetchpatch {
       url = "https://github.com/llvm-mirror/libunwind/commit/34a45c630d4c79af403661d267db42fbe7de1178.patch";
@@ -22,7 +24,7 @@ stdenv.mkDerivation {
     })
   ];
 
-  nativeBuildInputs = [ cmake llvm ];
+  nativeBuildInputs = [ cmake llvm.dev ];
 
   cmakeFlags = lib.optionals (!enableShared) [
     "-DLIBUNWIND_ENABLE_SHARED=OFF"