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/8/libunwind/default.nix

@@ -2,13 +2,11 @@
 , enableShared ? !stdenv.hostPlatform.isStatic
 }:
 
-stdenv.mkDerivation {
+stdenv.mkDerivation rec {
   pname = "libunwind";
   inherit version;
 
-  src = fetch "libunwind" "0vhgcgzsb33l83qaikrkj87ypqb48mi607rccczccwiiv8ficw0q";
-
-  nativeBuildInputs = [ cmake ];
+  src = fetch pname "0vhgcgzsb33l83qaikrkj87ypqb48mi607rccczccwiiv8ficw0q";
 
   patches = [
     (fetchpatch {
@@ -19,7 +17,12 @@ stdenv.mkDerivation {
       url = "https://github.com/llvm-mirror/libunwind/commit/e050272d2eb57eb4e56a37b429a61df2ebb8aa3e.patch";
       sha256 = "1sxyx5xnax8k713jjcxgq3jq3cpnxygs2rcdf5vfja0f2k9jzldl";
     })
+    ./gnu-install-dirs.patch
   ];
 
+  outputs = [ "out" "dev" ];
+
+  nativeBuildInputs = [ cmake ];
+
   cmakeFlags = lib.optional (!enableShared) "-DLIBUNWIND_ENABLE_SHARED=OFF";
 }

pkgs/development/compilers/llvm/8/libunwind/gnu-install-dirs.patch

@@ -0,0 +1,28 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 3d06073cfe74..55c7d1635fcd 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -18,6 +18,8 @@ set(CMAKE_MODULE_PATH
+ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+   project(libunwind)
+ 
++  include(GNUInstallDirs)
++
+   # Rely on llvm-config.
+   set(CONFIG_OUTPUT)
+   if(NOT LLVM_CONFIG_PATH)
+diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
+index f7523f88b6ac..4060dad214de 100644
+--- a/src/CMakeLists.txt
++++ b/src/CMakeLists.txt
+@@ -151,8 +151,8 @@ add_custom_target(unwind DEPENDS ${LIBUNWIND_BUILD_TARGETS})
+ 
+ if (LIBUNWIND_INSTALL_LIBRARY)
+   install(TARGETS ${LIBUNWIND_INSTALL_TARGETS}
+-    LIBRARY DESTINATION ${LIBUNWIND_INSTALL_PREFIX}lib${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind
+-    ARCHIVE DESTINATION ${LIBUNWIND_INSTALL_PREFIX}lib${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind)
++    LIBRARY DESTINATION ${LIBUNWIND_INSTALL_PREFIX}${CMAKE_INSTALL_LIBDIR}${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind
++    ARCHIVE DESTINATION ${LIBUNWIND_INSTALL_PREFIX}${CMAKE_INSTALL_LIBDIR}${LIBUNWIND_LIBDIR_SUFFIX} COMPONENT unwind)
+ endif()
+ 
+ if (NOT CMAKE_CONFIGURATION_TYPES AND LIBUNWIND_INSTALL_LIBRARY)