tcg: add the tcg-stub.c file into accel/stubs/

If tcg is disabled, the functions in tcg-stub.c file will be called.
This file is target-independent file, do not include any platform
related stub functions into this file.
Signed-off-by: NYang Zhong <yang.zhong@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

accel/stubs/Makefile.objs

 obj-$(call lnot,$(CONFIG_KVM)) += kvm-stub.o
+obj-$(call lnot,$(CONFIG_TCG)) += tcg-stub.o

accel/stubs/tcg-stub.c

+/*
+ * QEMU TCG accelerator stub
+ *
+ * Copyright Red Hat, Inc. 2013
+ *
+ * Author: Paolo Bonzini     <pbonzini@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "cpu.h"
+#include "tcg/tcg.h"
+#include "exec/cpu-common.h"
+#include "exec/exec-all.h"
+
+void tb_flush(CPUState *cpu)
+{
+}