[compiler-rt] Add support for Fuchsia

[phosek]

Summary

Fuchsia is a new operating system which uses compiler-rt builtins, this change adds support for cross-compiling compiler-rt builtins for Fuchsia.

[phosek]

Any comments or suggestions are welcome. I know that cross-compilation is still not fully supported with CMake, but it's how we currently build compiler-rt for Fuchsia. We are invoking CMake as (here is a concrete version):

cmake -DCMAKE_C_COMPILER=llvm-build/bin/clang -DLLVM_CONFIG_PATH=llvm-build/bin/llvm-config -DFUCHSIA=1 llvm-source/runtimes/compiler-rt/lib/builtins

This is very similar to how llvm/runtime/CMakeLists.txt builds compile-rt builtins, the only difference is the FUCHSIA=1 flag to enforce the cross-compilation.

[phosek]

Abandoned in favor of D26652 which is a more universal solution.

cmake/base-config-ix.cmake

if (LLVM_TREE_AVAILABLE)
   # Windows where we need to use clang-cl instead.
   if(NOT MSVC)
     set(COMPILER_RT_TEST_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang)
     set(COMPILER_RT_TEST_CXX_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++)
   else()
     set(COMPILER_RT_TEST_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang.exe)
     set(COMPILER_RT_TEST_CXX_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++.exe)
   endif()
+elseif(FUCHSIA)
+  execute_process(
+    COMMAND ${LLVM_CONFIG_PATH} "--version"
+    RESULT_VARIABLE HAD_ERROR
+    OUTPUT_VARIABLE CONFIG_OUTPUT)
+  if (HAD_ERROR)
+    message(FATAL_ERROR "llvm-config failed with status ${HAD_ERROR}")
+  endif()
+  string(REGEX MATCH "[0-9]+\\.[0-9]+(\\.[0-9]+)?" CLANG_VERSION
+         ${CONFIG_OUTPUT})
+  set(COMPILER_RT_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR})
+  set(COMPILER_RT_EXEC_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR}/bin)
+  set(COMPILER_RT_INSTALL_PATH ${CMAKE_INSTALL_PREFIX}/lib/clang/${CLANG_VERSION})
 else()
     # Take output dir and install path from the user.
   set(COMPILER_RT_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR} CACHE PATH
     "Path where built compiler-rt libraries should be stored.")
   set(COMPILER_RT_EXEC_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR}/bin CACHE PATH
     "Path where built compiler-rt executables should be stored.")
   set(COMPILER_RT_INSTALL_PATH ${CMAKE_INSTALL_PREFIX} CACHE PATH
     "Path where built compiler-rt libraries should be installed.")
if (MSVC)
     endif()
   endif()
 
   # Generate the COMPILER_RT_SUPPORTED_ARCH list.
   if(ANDROID)
     # Examine compiler output to determine target architecture.
     detect_target_arch()
     set(COMPILER_RT_OS_SUFFIX "-android")
+  elseif(FUCHSIA)
+    # Test the target architectures supported by the compiler.
+    test_target_arch(arm "" "--target=arm-fuchsia" "-march=armv7-a" "-mfloat-abi=soft")
+    test_target_arch(armhf "" "--target=arm-fuchsia" "-march=armv7-a" "-mfloat-abi=hard")
+    test_target_arch(x86_64 "" "--target=x86_64-fuchsia")
+    test_target_arch(aarch64 "" "--target=aarch64-fuchsia")
   elseif(NOT APPLE) # Supported archs for Apple platforms are generated later
     if("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "i[2-6]86|x86|amd64")
       if(NOT MSVC)
         test_target_arch(x86_64 "" "-m64")
         # FIXME: We build runtimes for both i686 and i386, as "clang -m32" may
         # target different variant than "$CMAKE_C_COMPILER -m32". This part should
         # be gone after we resolve PR14109.
         test_target_arch(i686 __i686__ "-m32")