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/lldb/gnu-install-dirs.patch

@@ -0,0 +1,81 @@
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index cdf22c4b0fc8..8def776f2b4a 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -7,6 +7,8 @@ set(CMAKE_MODULE_PATH
+   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
+   )
+ 
++include(GNUInstallDirs)
++
+ include(LLDBStandalone)
+ include(LLDBConfig)
+ include(AddLLDB)
+diff --git a/cmake/modules/AddLLDB.cmake b/cmake/modules/AddLLDB.cmake
+index f82c11d3d317..776e6d6ef9de 100644
+--- a/cmake/modules/AddLLDB.cmake
++++ b/cmake/modules/AddLLDB.cmake
+@@ -63,18 +63,18 @@ function(add_lldb_library name)
+             set(install_dir ".")
+           endif()
+         else()
+-          set(install_dir lib${LLVM_LIBDIR_SUFFIX})
++          set(install_dir ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX})
+         endif()
+         install(TARGETS ${name}
+           COMPONENT ${name}
+-          RUNTIME DESTINATION bin
++          RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+           LIBRARY DESTINATION ${install_dir}
+           ARCHIVE DESTINATION ${install_dir})
+       else()
+         install(TARGETS ${name}
+           COMPONENT ${name}
+-          LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX}
+-          ARCHIVE DESTINATION lib${LLVM_LIBDIR_SUFFIX})
++          LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX}
++          ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX})
+       endif()
+       if (NOT CMAKE_CONFIGURATION_TYPES)
+         add_llvm_install_targets(install-${name}
+@@ -122,7 +122,7 @@ function(add_lldb_executable name)
+   if(ARG_GENERATE_INSTALL)
+     install(TARGETS ${name}
+             COMPONENT ${name}
+-            RUNTIME DESTINATION bin)
++            RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+     if (NOT CMAKE_CONFIGURATION_TYPES)
+       add_llvm_install_targets(install-${name}
+                                DEPENDS ${name}
+diff --git a/cmake/modules/LLDBConfig.cmake b/cmake/modules/LLDBConfig.cmake
+index 109c5132d3be..8b9092faa29d 100644
+--- a/cmake/modules/LLDBConfig.cmake
++++ b/cmake/modules/LLDBConfig.cmake
+@@ -319,7 +319,7 @@ 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
+@@ -329,7 +329,7 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
+ 
+   install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/include/
+     COMPONENT lldb-headers
+-    DESTINATION include
++    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+     FILES_MATCHING
+     PATTERN "*.h"
+     PATTERN ".svn" EXCLUDE
+diff --git a/tools/intel-features/CMakeLists.txt b/tools/intel-features/CMakeLists.txt
+index b5316540fdf3..3c3c882d503f 100644
+--- a/tools/intel-features/CMakeLists.txt
++++ b/tools/intel-features/CMakeLists.txt
+@@ -64,4 +64,4 @@ if (NOT LLDB_DISABLE_PYTHON AND LLDB_BUILD_INTEL_PT)
+ endif()
+ 
+ install(TARGETS lldbIntelFeatures
+-  LIBRARY DESTINATION lib${LLVM_LIBDIR_SUFFIX})
++  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}${LLVM_LIBDIR_SUFFIX})