accel/tcg: Properly unlink a TB linked to itself

accel/tcg: Properly unlink a TB linked to itself

When we remove dest from orig's links, we lose the link
that we rely on later to reset links.  This can lead to
failure to release from spinlock with self-modifying code.

Cc: qemu-stable@nongnu.org
Reported-by: 李威威 <liweiwei@kubuds.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Tested-by: Anton Johansson <anjo@rev.ng>