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/11/lldb/gnu-install-dirs.patch

@@ -0,0 +1,65 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index bf748020ea40..34103d2b5e1a 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -17,6 +17,8 @@ set(CMAKE_MODULE_PATH
+ # If we are not building as part of LLVM, build LLDB as a standalone project,
+ # using LLVM as an external library.
+ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
++  include(GNUInstallDirs)
++
+   project(lldb)
+   include(LLDBStandalone)
+ 
+diff --git a/cmake/modules/AddLLDB.cmake b/cmake/modules/AddLLDB.cmake
+index 4ed5c647c5d2..89f96e710d55 100644
+--- a/cmake/modules/AddLLDB.cmake
++++ b/cmake/modules/AddLLDB.cmake
+@@ -107,13 +107,13 @@ function(add_lldb_library name)
+   endif()
+ 
+   if(PARAM_SHARED)
+-    set(install_dest lib${LLVM_LIBDIR_SUFFIX})
++    set(install_dest ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX})
+     if(PARAM_INSTALL_PREFIX)
+       set(install_dest ${PARAM_INSTALL_PREFIX})
+     endif()
+     # RUNTIME is relevant for DLL platforms, FRAMEWORK for macOS
+     install(TARGETS ${name} COMPONENT ${name}
+-      RUNTIME DESTINATION bin
++      RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+       LIBRARY DESTINATION ${install_dest}
+       ARCHIVE DESTINATION ${install_dest}
+       FRAMEWORK DESTINATION ${install_dest})
+diff --git a/cmake/modules/LLDBConfig.cmake b/cmake/modules/LLDBConfig.cmake
+index 8465cfe3b7b7..01b7eae136e2 100644
+--- a/cmake/modules/LLDBConfig.cmake
++++ b/cmake/modules/LLDBConfig.cmake
+@@ -245,7 +245,7 @@ include_directories(BEFORE
+ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+   install(DIRECTORY include/
+     COMPONENT lldb-headers
+-    DESTINATION include
++    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+     FILES_MATCHING
+     PATTERN "*.h"
+     PATTERN ".svn" EXCLUDE
+@@ -254,7 +254,7 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+ 
+   install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/include/
+     COMPONENT lldb-headers
+-    DESTINATION include
++    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+     FILES_MATCHING
+     PATTERN "*.h"
+     PATTERN ".svn" EXCLUDE
+diff --git a/tools/intel-features/CMakeLists.txt b/tools/intel-features/CMakeLists.txt
+index efba2f74904f..e08413b1dbb1 100644
+--- a/tools/intel-features/CMakeLists.txt
++++ b/tools/intel-features/CMakeLists.txt
+@@ -65,4 +65,4 @@ if (LLDB_ENABLE_PYTHON AND LLDB_BUILD_INTEL_PT)
+ endif()
+ 
+ install(TARGETS lldbIntelFeatures
+-  LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX})
++  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX})