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: I558441a26a7c8390aa087f32c4cbe980de8c8ce3
Signed-off-by: Thierry Strudel <tstrudel@google.com>
Signed-off-by: Jeff Vander Stoep <jeffv@google.com>