build: Add support for ARM64 to the CMake build

Add support for ARM64 as a target architecture for builds.  Windows ARM64 should be possible to support, though calling convention issues may cause troubles at the moment.

Author: efalloon

cmake/Modules/SwiftSupport.cmake

@@ -29,6 +29,8 @@ function(get_swift_host_arch result_var_name)
     set("${result_var_name}" "armv7" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "armv7-a")
     set("${result_var_name}" "armv7" PARENT_SCOPE)
+  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ARM64")
+    set("${result_var_name}" "x86_64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "AMD64")
     set("${result_var_name}" "x86_64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "IA64")