native: use pref.arch to host_arch() by default

vlib/v/builder/native.v

@@ -32,6 +32,9 @@ pub fn (mut b Builder) build_native(v_files []string, out_file string) {
 pub fn (mut b Builder) compile_native() {
 	// v.files << v.v_files_from_dir(os.join_path(v.pref.vlib_path,'builtin','bare'))
 	files := [b.pref.path]
+	if b.pref.arch == ._auto {
+		b.pref.arch = pref.get_host_arch()
+	}
 	b.set_module_lookup_paths()
 	b.build_native(files, b.pref.out_name)
 }