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>
(cherry picked from commit 03fe6659803f83690b8587d01f8ee56bb4be4b90)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>