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/rust/1_45.nix

@@ -20,9 +20,9 @@ import ./default.nix {
   rustcVersion = "1.45.2";
   rustcSha256 = "0273a1g3f59plyi1n0azf21qjzwml1yqdnj5z472crz37qggr8xp";
 
-  llvmSharedForBuild = pkgsBuildBuild.llvm_10.override { enableSharedLibraries = true; };
-  llvmSharedForHost = pkgsBuildHost.llvm_10.override { enableSharedLibraries = true; };
-  llvmSharedForTarget = pkgsBuildTarget.llvm_10.override { enableSharedLibraries = true; };
+  llvmSharedForBuild = pkgsBuildBuild.llvmPackages_10.libllvm.override { enableSharedLibraries = true; };
+  llvmSharedForHost = pkgsBuildHost.llvmPackages_10.libllvm.override { enableSharedLibraries = true; };
+  llvmSharedForTarget = pkgsBuildTarget.llvmPackages_10.libllvm.override { enableSharedLibraries = true; };
 
   llvmBootstrapForDarwin = llvmPackages_5;
 

pkgs/development/compilers/rust/1_51.nix

@@ -22,9 +22,9 @@ import ./default.nix {
   rustcVersion = "1.51.0";
   rustcSha256 = "0ixqkqglv3isxbvl4ldr4byrkx692wghsz3fasy1pn5kr2prnsvs";
 
-  llvmSharedForBuild = pkgsBuildBuild.llvm_11.override { enableSharedLibraries = true; };
-  llvmSharedForHost = pkgsBuildHost.llvm_11.override { enableSharedLibraries = true; };
-  llvmSharedForTarget = pkgsBuildTarget.llvm_11.override { enableSharedLibraries = true; };
+  llvmSharedForBuild = pkgsBuildBuild.llvmPackages_11.libllvm.override { enableSharedLibraries = true; };
+  llvmSharedForHost = pkgsBuildHost.llvmPackages_11.libllvm.override { enableSharedLibraries = true; };
+  llvmSharedForTarget = pkgsBuildTarget.llvmPackages_11.libllvm.override { enableSharedLibraries = true; };
 
   llvmBootstrapForDarwin = llvmPackages_11;