gcc-plugins: add support plugin for arm64 per-task stack canaries

By default, GCC for AArch64 uses a global variable __stack_chk_guard
which is shared between all CPUs, and hence between all tasks, which
means the stack canary value only changes between reboots.

This plugin renames the symbol to __stack_chk_guard_tsk_offset, and
adds an RTL pass to replace the first instruction in each adrp/add pair
referring to the symbol with a load of sp_el0, which holds the address
of 'current'. In a subsequent patch, we will tweak the kernel build to
expose the offset of the stack_canary field in task_struct via the
symbol, so that the resulting reference resolves to the task's unique
canary value directly.

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