Add arm64 support to CMake build (apple#64)

This is pretty speculative, I just copied the `SwiftSupport.cmake` from swift-argument-parser which seems to have support for arm64.

Author: neonichu

cmake/modules/SwiftSupport.cmake

@@ -17,8 +17,12 @@ See https://swift.org/LICENSE.txt for license information
 function(get_swift_host_arch result_var_name)
   if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64")
     set("${result_var_name}" "x86_64" PARENT_SCOPE)
-  elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "aarch64")
-    set("${result_var_name}" "aarch64" PARENT_SCOPE)
+  elseif ("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "AArch64|arm64")
+    if(CMAKE_SYSTEM_NAME MATCHES Darwin)
+      set("${result_var_name}" "arm64" PARENT_SCOPE)
+    else()
+      set("${result_var_name}" "aarch64" PARENT_SCOPE)
+    endif()
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")
     set("${result_var_name}" "powerpc64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")