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/6/compiler-rt/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation {
   inherit version;
   src = fetch "compiler-rt" "1fcr3jn24yr8lh36nc0c4ikli4744i2q9m1ik67p1jymwwaixkgl";
 
-  nativeBuildInputs = [ cmake python3 llvm ];
+  nativeBuildInputs = [ cmake python3 llvm.dev ];
   buildInputs = lib.optional stdenv.hostPlatform.isDarwin libcxxabi;
 
   NIX_CFLAGS_COMPILE = [
@@ -21,6 +21,7 @@ stdenv.mkDerivation {
   ];
 
   cmakeFlags = [
+    "-DCOMPILER_RT_OS_DIR="
     "-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON"
     "-DCMAKE_C_COMPILER_TARGET=${stdenv.hostPlatform.config}"
     "-DCMAKE_ASM_COMPILER_TARGET=${stdenv.hostPlatform.config}"
@@ -54,6 +55,7 @@ stdenv.mkDerivation {
     ./codesign.patch # Revert compiler-rt commit that makes codesign mandatory
     # https://github.com/llvm/llvm-project/commit/947f9692440836dcb8d88b74b69dd379d85974ce
     ../../common/compiler-rt/glibc.patch
+    ./gnu-install-dirs.patch
   ] ++ lib.optional stdenv.hostPlatform.isMusl ./sanitizers-nongnu.patch
     ++ lib.optional stdenv.hostPlatform.isAarch32 ./armv7l.patch;
 
@@ -78,9 +80,7 @@ stdenv.mkDerivation {
   '';
 
   # Hack around weird upsream RPATH bug
-  postInstall = lib.optionalString (stdenv.hostPlatform.isDarwin || stdenv.hostPlatform.isWasm) ''
-    ln -s "$out/lib"/*/* "$out/lib"
-  '' + lib.optionalString (useLLVM) ''
+  postInstall = lib.optionalString (useLLVM) ''
     ln -s $out/lib/*/clang_rt.crtbegin-*.o $out/lib/linux/crtbegin.o
     ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/linux/crtend.o
     ln -s $out/lib/*/clang_rt.crtbegin_shared-*.o $out/lib/linux/crtbeginS.o