Add arm64_32 support for Swift

Commit the platform definition and build script work necessary to
cross-compile for arm64_32.

arm64_32 is a variant of AARCH64 that supports an ILP32 architecture.

lib/Basic/Platform.cpp

@@ -262,6 +262,7 @@ getArchForAppleTargetSpecificModuleTriple(const llvm::Triple &triple) {
   //          .Case ("armv7s", "armv7s")
   //          .Case ("armv7k", "armv7k")
   //          .Case ("armv7", "armv7")
+  //          .Case ("arm64_32", "arm64_32")
   //          .Case ("arm64e", "arm64e")
               .Default(tripleArchName);
 }