[CMake][Fuchsia] Build runtimes as universal libraries on OS X

[phosek]

Summary

We want to build libunwind, libc++abi and libc++ as universal libraries
supporting both x86_64 and arm64 architectures.

clang/cmake/caches/Fuchsia-stage2.cmake

if(APPLE)
 
   set(LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
   set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
   set(LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
   set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
-  set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
+  set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
-  set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
+  set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
+  set(RUNTIMES_CMAKE_ARGS "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.13;-DCMAKE_OSX_ARCHITECTURES=arm64|x86_64" CACHE STRING "")
 endif()
 
 if(WIN32)
   set(target "x86_64-pc-windows-msvc")
 
   list(APPEND BUILTIN_TARGETS "${target}")
   set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(BUILTINS_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")

clang/cmake/caches/Fuchsia.cmake

else()
   set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXXABI_ENABLE_STATIC_UNWINDER ON CACHE BOOL "")
   set(LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXXABI_USE_COMPILER_RT ON CACHE BOOL "")
   set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
+  set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
+  set(RUNTIMES_CMAKE_ARGS "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.13;-DCMAKE_OSX_ARCHITECTURES=arm64|x86_64" CACHE STRING "")
 endif()
 
 if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
   set(target "${BOOTSTRAP_CMAKE_CXX_COMPILER_TARGET}")
   if(STAGE2_LINUX_${target}_SYSROOT)
     set(LLVM_BUILTIN_TARGETS "${target}" CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")
     set(BUILTINS_${target}_CMAKE_BUILD_TYPE Release CACHE STRING "")