ARM: ensure the signal page contains defined contents

stable inclusion
from stable-5.10.17
commit 7913ec05fc02ccd7df83280451504b0a3e543097
bugzilla: 48169

--------------------------------

[ Upstream commit 9c698bff ]

Ensure that the signal page contains our poison instruction to increase
the protection against ROP attacks and also contains well defined
contents.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>