add stub routines stackalloc() and stackfree().

run oldstack on g0's stack, just like newstack does,
so that oldstack can free the old stack.

R=r
DELTA=53  (44 added, 0 deleted, 9 changed)
OCL=20404
CL=20433

src/runtime/stack.c

@@ -0,0 +1,19 @@
+// Copyright 2009 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+
+// Stubs for stack management.
+// In a separate file so they can be overridden during testing of gc.
+
+void*
+stackalloc(uint32 n)
+{
+	return mal(n);
+}
+
+void
+stackfree(void*)
+{
+}