arm64: Mark __stack_chk_guard as __ro_after_init

arm64: Mark __stack_chk_guard as __ro_after_init

__stack_chk_guard is setup once while init stage and never changed
after that.

Although the modification of this variable at runtime will usually
cause the kernel to crash (so does the attacker), it should be marked
as __ro_after_init, and it should not affect performance if it is
placed in the ro_after_init section.

Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/1631612642-102881-1-git-send-email-ashimida@linux.alibaba.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>