ARM: 8789/1: signal: copy registers using __copy_to_user()

When saving the ARM integer registers, use __copy_to_user() to
copy them into user signal frame, rather than __put_user_error().
This has the benefit of disabling/enabling PAN once for the whole copy
intead of once per write.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>