[cmake] WIP: Add cache file to bootstrap linux cross compile on Darwin.

[hintonda]

Summary

Add ability to cross compile clang on Darwin and target Linux.

This patch has the following issues:

• LIbFuzzer fails to compile due to issues with compiling an internal version of libc++.
  • Patch: TBD
  • Workaround: pass -DBOOTSTRAP_COMPILER_RT_BUILD_LIBFUZZER=OFF

[hintonda]

• Also pass LLVM_CONFIG_EXE.

[hintonda]

Refactor to eliminate need for second stage and toolchain files.

[hintonda]

• Also pass LLVM_CONFIG_EXE.
• Add llvm-objcopy, and make sure that is stage2 cache file is passed,

[hintonda]

• Use LLVM_CONFIG_PATH instead of LLVM_CONFIG_EXE.

[hintonda]

• Libfuzzers won't build, so turn off.
• Fix typos, and set cache.

cmake/caches/Linux.cmake

+# This file is primarily for cross compiling clang+llvm, et al, for
+# Linux on Darwin, and can be invoked like this:
+#
+#  cmake -GNinja -DBOOTSTRAP_CMAKE_SYSROOT=<sysroot path> [OPTIONS] \
+#    -C <clang root>/cmake/caches/Linux.cmake ../llvm
+#
+#  Use "BOOTSTRAP_" prefix to pass options to stage2, e.g.,
+#  "-DBOOTSTRAP_CMAKE_BUILD_TYPE=Release", and
+#  "-DSTAGE2_CACHE_FILE=<path>" to specify a stage2 cache file.
+#
+
+
+if(NOT DEFINED BOOTSTRAP_CMAKE_SYSROOT)
+  message(FATAL_ERROR "Missing required argument -DBOOTSTRAP_CMAKE_SYSROOT=<sysroot path>.")
+endif()
+
+# FIXME: Default to Linux, but should this be required?
+if(NOT DEFINED BOOTSTRAP_CMAKE_SYSTEM_NAME)
+  set(BOOTSTRAP_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
+endif()
+
+# FIXME:  Should this default be based on BOOTSTRAP_CMAKE_SYSTEM_NAME?
+if(NOT DEFINED BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE)
+  set(BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE "x86_64-unknown-linux-gnu" CACHE STRING "")
+endif()
+
+if(NOT DEFINED BOOTSTRAP_GCC_INSTALL_PREFIX)
+  # Force clang to look for gcc at runtime -- otherwise it will
+  # default to 4.2.1.
+  set(BOOTSTRAP_GCC_INSTALL_PREFIX "/usr" CACHE STRING "")
+endif()
+
+# Allow user to pass a custom stage2 cache file.
+if(DEFINED STAGE2_CACHE_FILE)
+  if(NOT IS_ABSOLUTE ${STAGE2_CACHE_FILE})
+    message(SEND_ERROR "STAGE2_CACHE_FILE [${STAGE2_CACHE_FILE}] must be an absolute path.")
+  endif()
+  if(NOT EXISTS ${STAGE2_CACHE_FILE})
+    message(SEND_ERROR "STAGE2_CACHE_FILE [${STAGE2_CACHE_FILE}] does not exist.")
+  endif()
+  list(APPEND EXTRA_ARGS -C${STAGE2_CACHE_FILE})
+endif()
+
+list(APPEND EXTRA_ARGS
+  -DCMAKE_C_COMPILER_TARGET=${BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE}
+  -DCMAKE_CXX_COMPILER_TARGET=${BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE}
+  -DCMAKE_ASM_COMPILER_TARGET=${BOOTSTRAP_LLVM_DEFAULT_TARGET_TRIPLE}
+
+  # Adjust the default behaviour of the FIND_XXX() commands to only
+  # search for headers and libraries in the
+  # CMAKE_SYSROOT/CMAKE_FIND_ROOT_PATH, and always search for programs
+  # in the host system.  These all default to BOTH.
+  -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER
+  -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY
+  -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY
+  )
+
+set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
+set(CMAKE_BUILD_TYPE RELEASE CACHE STRING "")
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+
+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 "")
+
+# FIXME: Is there a better way to specify this? i.e., target is elf,
+# but host isn't.
+if(CMAKE_HOST_APPLE)
+  # Make sure at least clang and lld are included.
+  set(LLVM_ENABLE_PROJECTS ${LLVM_ENABLE_PROJECTS} clang lld CACHE STRING "")
+
+  # Passing -fuse-ld=lld is hard for cmake to handle correctly, so
+  # make lld the default linker.
+  set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
+
+  set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
+
+  list(APPEND EXTRA_ARGS
+    # FIXME: Libfuzzer won't build correctly, so turn off for now.
+    -DCOMPILER_RT_BUILD_LIBFUZZER=OFF
+    -DLLVM_ENABLE_LLD=ON
+    -DCMAKE_AR=${CMAKE_BINARY_DIR}/bin/llvm-ar
+    -DCMAKE_OBJCOPY=${CMAKE_BINARY_DIR}/bin/llvm-objcopy
+    -DCMAKE_RANLIB=${CMAKE_BINARY_DIR}/bin/llvm-ranlib
+    -DCLANG_TABLEGEN=${CMAKE_BINARY_DIR}/bin/clang-tblgen
+    -DLLVM_TABLEGEN=${CMAKE_BINARY_DIR}/bin/llvm-tblgen
+    -DLLVM_CONFIG_PATH=${CMAKE_BINARY_DIR}/bin/llvm-config
+    )
+
+endif()
+
+set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
+set(CLANG_BOOTSTRAP_CMAKE_ARGS ${EXTRA_ARGS} CACHE STRING "")