[CMake][Fuchsia] Start building arm64 Darwin runtimes

This enables arm64 Darwin support in Fuchsia toolchain.

Author: petrhosek

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -69,7 +69,7 @@ if(APPLE)
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(DARWIN_ios_ARCHS armv7;armv7s;arm64 CACHE STRING "")
   set(DARWIN_iossim_ARCHS i386;x86_64 CACHE STRING "")
-  set(DARWIN_osx_ARCHS x86_64 CACHE STRING "")
+  set(DARWIN_osx_ARCHS arm64;x86_64 CACHE STRING "")
   set(SANITIZER_MIN_OSX_VERSION 10.7 CACHE STRING "")
 endif()