Make sure we get typealias Float16 = Half when we need it

Author: dabrahams

Sources/CBORCoding/Half.swift

@@ -1,6 +1,7 @@
 import Foundation
 
-#if canImport(Half) && swift(<5.3)
+#if canImport(Half) && swift(<5.3) || (
+  os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64)
 import Half
 
 typealias Float16 = Half