libstdc++: Fix 29_atomics/headers/atomic/types_std_c++2a_neg.cc for C++23

This test fails when run as C++23 because the <bits/stdc++.h> PCH
includes <stdatomic.h> which declares ::memory_order, invalidating the
test's assumptions. Disable PCH so that the test verifies that <atomic>
doesn't declare ::memory_order, as originally intended.

Also fix the using-declaration which would be invalid even if the type
was declared in the global namespace, and adjust the expected error.

libstdc++-v3/ChangeLog:

	* testsuite/29_atomics/headers/atomic/types_std_c++2a_neg.cc:
	Fix test to work for C++23 and C++26 too.

libstdc++-v3/testsuite/29_atomics/headers/atomic/types_std_c++2a_neg.cc

 // { dg-options "-std=gnu++2a" }
-// { dg-do compile { target c++2a } }
+// { dg-do compile { target c++20 } }
+// Disable PCH because <bits/stdc++.h> includes <stdatomic.h> which declares
+// memory_order in the global namespace.
+// { dg-add-options no_pch }
 
 // Copyright (C) 2019-2023 Free Software Foundation, Inc.
 //
@@ -23,7 +26,7 @@
 void test01()
 {
   // Not global scoped, only namespace std.
-  using memory_order; // { dg-error "expected nested-name-specifier" }
+  using ::memory_order; // { dg-error "has not been declared" }
   constexpr auto relaxed = memory_order::relaxed; // { dg-error "has not been declared" }
   constexpr auto consume = memory_order::consume; // { dg-error "has not been declared" }
   constexpr auto acquire = memory_order::acquire; // { dg-error "has not been declared" }