arm64: kernel: use a unique stack canary value for each task

Enable the support plugin and expose an appropriate value for
__stack_chk_guard_tsk_offset so that function prologues and
epilogues emitted by GCC read the stack canary value straight
from the task_struct. This sidesteps any concurrency issues
resulting from the use of per-CPU variables to store the canary
value of the currently running task.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>