arm64 Kconfig: Move LIST_POISON to a safe value

Move the poison pointer offset to 0xdead000000000000, a
recognized value that is not mappable by user-space exploits.

Change-Id: I1ddae985f2fc28ca1366747169d4ca91801a9d31
Signed-off-by: Thierry Strudel <tstrudel@google.com>
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>