Add support for Apple Silicon in SwiftSupport.cmake (#257)

This matches the behavior of [a similar `elseif` branch in the Swift toolchain itself](https://github.com/apple/swift/blob/b80e4bf8f866be26a4108bd92f97b2a182662642/CMakeLists.txt#L594).

Author: MaxDesiatov

cmake/modules/SwiftSupport.cmake

@@ -10,6 +10,8 @@ function(get_swift_host_arch result_var_name)
     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}" STREQUAL "arm64")
+    set("${result_var_name}" "aarch64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")
     set("${result_var_name}" "powerpc64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")