sync/atomic: allow linked list of atomic pointers again

For #56603, CL 448275 added a _ [0]T field to atomic.Pointer,
so that different kinds of atomic.Pointer are not convertible.

Unfortunately, that breaks code like:

	type List struct {
		Next atomic.Pointer[List]
	}

which should be valid, just as using Next *List is valid.
Instead, we get:

	./atomic_test.go:2533:6: invalid recursive type List
		./atomic_test.go:2533:6: List refers to
		./atomic_test.go:2534:13: "sync/atomic".Pointer refers to
		./atomic_test.go:2533:6: List

Fix by using _[0]*T instead.

Change-Id: Icc4c83c691d35961d20cb14b824223d6c779ac5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/450655
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/sync/atomic/atomic_test.go

@@ -2526,3 +2526,9 @@ func TestNilDeref(t *testing.T) {
 		}()
 	}
 }
+
+// Test that this compiles.
+// When atomic.Pointer used _ [0]T, it did not.
+type List struct {
+	Next Pointer[List]
+}

src/sync/atomic/type.go

@@ -41,9 +41,10 @@ var _ = &Pointer[int]{}
 
 // A Pointer is an atomic pointer of type *T. The zero value is a nil *T.
 type Pointer[T any] struct {
-	// Mention T in a field to disallow conversion between Pointer types.
+	// Mention *T in a field to disallow conversion between Pointer types.
 	// See go.dev/issue/56603 for more details.
-	_ [0]T
+	// Use *T, not T, to avoid spurious recursive type definition errors.
+	_ [0]*T
 
 	_ noCopy
 	v unsafe.Pointer