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/12/lldb/resource-dir.patch

@@ -0,0 +1,13 @@
+diff --git a/lldb/cmake/modules/LLDBConfig.cmake b/lldb/cmake/modules/LLDBConfig.cmake
+index 37364341ff8b..7f74c1a3e257 100644
+--- a/cmake/modules/LLDBConfig.cmake
++++ b/cmake/modules/LLDBConfig.cmake
+@@ -257,7 +257,7 @@ if (NOT TARGET clang-resource-headers)
+   # Iterate over the possible places where the external resource directory
+   # could be and pick the first that exists.
+   foreach(CANDIDATE "${Clang_DIR}/../.." "${LLVM_DIR}" "${LLVM_LIBRARY_DIRS}"
+-                    "${LLVM_BUILD_LIBRARY_DIR}"
++                    "${LLVM_BUILD_LIBRARY_DIR}" "@clangLibDir@"
+                     "${LLVM_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}")
+     # Build the resource directory path by appending 'clang/<version number>'.
+     set(CANDIDATE_RESOURCE_DIR "${CANDIDATE}/clang/${LLDB_CLANG_RESOURCE_DIR_NAME}")