BACKPORT: ARM: allow kuser helpers to be removed from the vector page

(cherry pick from commit f6f91b0d)

Provide a kernel configuration option to allow the kernel user helpers
to be removed from the vector page, thereby preventing their use with
ROP (return orientated programming) attacks.  This option is only
visible for CPU architectures which natively support all the operations
which kernel user helpers would normally provide, and must be enabled
with caution.

Cc: <stable@vger.kernel.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Bug: 9674955
Bug: 25773781
Change-Id: I3a692024cdab44db2b5d3b533e61bd11a904836c