[cmake] Recognize "arm64" as a supported arch

On macOS with ARM processors, CMAKE_SYSTEM_PROCESSOR is set to "arm64",
which was not properly recognized as an "aarch64" architecture.

Author: Steelskin

cmake/modules/SwiftSupport.cmake

@@ -19,6 +19,8 @@ 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 "ARM64")
     set("${result_var_name}" "aarch64" PARENT_SCOPE)
   elseif("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64")