Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -550,6 +550,10 @@
     add_dependencies(clang-bootstrap-deps lld)
   endif()
 
+  if(ADDITIONAL_CLANG_BOOTSTRAP_DEPS)
+    add_dependencies(clang-bootstrap-deps ${ADDITIONAL_CLANG_BOOTSTRAP_DEPS})
+  endif()
+
   # If the next stage is LTO we need to depend on LTO and possibly lld or LLVMgold
   if(BOOTSTRAP_LLVM_ENABLE_LTO OR LLVM_ENABLE_LTO AND NOT LLVM_BUILD_INSTRUMENTED)
     if(APPLE)
Index: cmake/caches/linux-toolchain.cmake
===================================================================
--- /dev/null
+++ cmake/caches/linux-toolchain.cmake
@@ -0,0 +1,142 @@
+# This file is intended to support cross compiling a linux toolchain
+# on any host system, includind Darwin.
+#
+# Usage: cmake -GNinja -DSYSROOT=<path> [OPTIONS] -C ../clang/cmake/caches/linux-toolchain.cmake  ../llvm
+#
+#   OPTIONS:
+#     Regular options apply to stage1.
+#     BOOTSTRAP_ options apply to stage2, and should use the BOOTSTRAP_ prefix.
+#
+# Then run "ninja stage2" to cross compile.  Bins and libs can be
+# found in tools/clang/stage2-bins.
+#
+# Known issues:
+#
+#  1) This toolchain assumes a flat, mono-repo layout.
+#
+#  2) Several sub-projects, including libcxx, libcxxabi, and
+#     libunwind, use FIND_PATH() to find the source path for headers
+#     of other sub-projects on the host system, but fail because they
+#     don't specify NO_CMAKE_FIND_ROOT_PATH.  The following pathes are
+#     reqired to build these projects:
+#
+#       libcxx       https://reviews.llvm.org/D41622
+#       libcxxabi    https://reviews.llvm.org/D41623
+#       libunwind    https://reviews.llvm.org/D41621
+#
+#  3) Libraries in the compiler-rt sub-project fail to link with the
+#     following error:
+#
+#       bin/ld.lld: error:
+#       projects/compiler-rt/lib/sanitizer_common/CMakeFiles/RTSanitizerCommon.x86_64.dir/sanitizer_linux_x86_64.S.o:
+#       invalid data encoding
+#
+#  4) FIND_PACKAGE can fail if the package file uses PkgConfig, e.g.,
+#     FindLibXml2.cmake, since the pkg-config program runs on the
+#     host, and the variables set by PKG_CHECK_MODULES reference the
+#     host system, not the target.
+#
+#  5) Stage2 configuration fails for several sub-projects, including
+#     libcxx, libcxxabi, and libunwind, with the following error:
+#
+#       CMake Error at .../llvm_project/libunwind/src/CMakeLists.txt:110 (add_library):
+#       The install of the unwind_shared target requires changing an
+#       RPATH from the build tree, but this is not supported with the
+#       Ninja generator unless on an ELF-based platform.  The
+#       CMAKE_BUILD_WITH_INSTALL_RPATH variable may be set to avoid
+#       this relinking step.
+#
+
+if(NOT DEFINED SYSROOT AND NOT DEFINED CMAKE_SYSROOT)
+  message(FATAL_ERROR "Missing required option -DSYSROOT=<sysroot path>.")
+endif()
+
+if(DEFINED SYSROOT)
+  # Since we just want to build a bootstrap compiler, turn off as much
+  # as possible.
+  set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
+  set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
+  set(LLVM_INCLUDE_RUNTIMES OFF CACHE BOOL "")
+  set(LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
+  set(LLVM_INCLUDE_UTILS OFF CACHE BOOL "")
+  set(CLANG_BUILD_TOOLS OFF CACHE BOOL "")
+  set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
+  set(CLANG_ENABLE_STATIC_ANALYZER OFF CACHE BOOL "")
+
+  set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
+  set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
+  set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+
+  # Make sure at least clang and lld are included.
+  list(APPEND LLVM_ENABLE_PROJECTS clang lld)
+  set(LLVM_ENABLE_PROJECTS ${LLVM_ENABLE_PROJECTS} CACHE STRING "")
+
+  # Passing -fuse-ld=lld is hard for cmake to handle correctly, so
+  # make lld the default linker.
+  set(CLANG_DEFAULT_LINKER lld CACHE STRING "" FORCE)
+
+  # Since LLVM_ENABLE_PROJECTS gets passed automatically to the next
+  # stage, use another variable to pass the desired projects to stage2.
+  if(DEFINED BOOTSTRAP_LLVM_ENABLE_PROJECTS)
+    set(BOOTSTRAP_STAGE2_PROJECTS ${BOOTSTRAP_LLVM_ENABLE_PROJECTS} CACHE STRING "" FORCE)
+    unset(BOOTSTRAP_LLVM_ENABLE_PROJECTS CACHE)
+  else()
+    set(BOOTSTRAP_STAGE2_PROJECTS "clang;libcxx;libcxxabi;libunwind" CACHE STRING "" FORCE)
+  endif()
+
+  # Required on non-elf hosts.
+  set(ADDITIONAL_CLANG_BOOTSTRAP_DEPS "lld;llvm-ar;llvm-ranlib" CACHE STRING "")
+
+  set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
+  set(CLANG_BOOTSTRAP_CMAKE_ARGS
+    -DCMAKE_SYSROOT=${SYSROOT}
+    -DCMAKE_TOOLCHAIN_FILE=${CMAKE_CURRENT_LIST_FILE} CACHE STRING "")
+else()
+  set(CMAKE_SYSTEM_NAME Linux CACHE STRING "" FORCE)
+  set(LLVM_DEFAULT_TARGET_TRIPLE "x86_64-unknown-linux-gnu" CACHE STRING "" FORCE)
+
+  # Set default, but allow overries.
+  set(CMAKE_BUILD_TYPE Release CACHE STRING "")
+
+  # Always use STAGE2_PROJECTS Use FORCE to override passed vaiable.
+  set(LLVM_ENABLE_PROJECTS ${STAGE2_PROJECTS} CACHE STRING "" FORCE)
+
+  # Use bootstrap tools.
+  get_filename_component(BASE_PATH "${CMAKE_C_COMPILER}" DIRECTORY CACHE)
+  set(CMAKE_AR "${BASE_PATH}/llvm-ar" CACHE STRING "")
+  set(CMAKE_RANLIB "${BASE_PATH}/llvm-ranlib" CACHE STRING "")
+  set(CLANG_TABLEGEN "${BASE_PATH}/clang-tblgen" CACHE STRING "")
+  set(LLVM_TABLEGEN "${BASE_PATH}/llvm-tblgen" CACHE STRING "")
+  set(_LLVM_CONFIG_EXE "${BASE_PATH}/llvm-config" CACHE STRING "")
+  set(CMAKE_LINKER "${BASE_PATH}/lld" CACHE STRING "")
+
+  # The compiler used in stage2 needs both -target and --sysroot.
+  # Setting CMAKE_SYSROOT handles the --sysroot option, but this is
+  # the only way to consistently pass the -target option.
+  set(CMAKE_C_COMPILER_ARG1 "-target ${LLVM_DEFAULT_TARGET_TRIPLE}" CACHE STRING "" FORCE)
+  set(CMAKE_CXX_COMPILER_ARG1 "-target ${LLVM_DEFAULT_TARGET_TRIPLE}" CACHE STRING "" FORCE)
+
+  # Make sure static libs use the gnu format.
+  set(CMAKE_STATIC_LINKER_FLAGS "-format gnu" CACHE STRING "")
+
+  # Force clang to look for gcc lookup at runtime -- otherwise it will
+  # default to 4.2.1.
+  set(GCC_INSTALL_PREFIX "/usr" CACHE STRING "")
+
+  # Changing an RPATH from the build tree is not supported with the
+  # Ninja generator unless on an ELF-based platform.  This might
+  # require changes to llvm cmake files at some point.
+  if(CMAKE_HOST_APPLE AND CMAKE_GENERATOR STREQUAL "Ninja")
+    set(CMAKE_BUILD_WITH_INSTALL_RPATH ON CACHE BOOL "")
+  endif()
+
+  # Use CMAKE_SYSROOT prefix for FIND_XXX() commands.
+  SET(CMAKE_FIND_ROOT_PATH "${CMAKE_SYSROOT}" CACHE STRING "")
+
+  # Adjust the default behaviour of the FIND_XXX() commands:
+  # search headers and libraries in the target environment, search
+  # programs in the host environment.
+  set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER CACHE STRING "")
+  set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY CACHE STRING "")
+  set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY CACHE STRING "")
+endif()