add arm64 support for M1 Mac

Author: DanboDuan

cmake/modules/SwiftSupport.cmake

@@ -9,6 +9,8 @@
 function(get_swift_host_arch result_var_name)
   if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64")
     set("${result_var_name}" "x86_64" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "arm64")
+    set("${result_var_name}" "arm64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "aarch64")
     set("${result_var_name}" "aarch64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")