ARM: poison the vectors page

Fill the empty regions of the vectors page with an exception generating
instruction.  This ensures that any inappropriate branch to the vector
page is appropriately trapped, rather than just encountering some code
to execute.  (The vectors page was filled with zero before, which
corresponds with the "andeq r0, r0, r0" instruction - a no-op.)

Change-Id: I74c0105a2bcb3b78d9fc8b105a05c0fc269ba400
Cc: <stable@vger.kernel.org>
Acked-by Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>