tests: fix test build
The DISPATCH_SIZEOF_PTR caused a test to go down the wrong path.  This
fixes the test and accounts for the Windows 64 bit case as well (which
is not a LP64 environment).
@@ -50,7 +50,7 @@ main(void)
{
dispatch_test_start("Dispatch Queue Finalizer");
-#if DISPATCH_SIZEOF_PTR == 8
+#if defined(__LP64__) || defined(_WIN64)
ctxt_magic = (void*)((uintptr_t)arc4random() << 32 | arc4random());
#else
ctxt_magic = (void*)arc4random();
