[AArch64] Support customizing stack protector guard

Follow up to D88631 but for aarch64; the Linux kernel uses the command
line flags:

1. -mstack-protector-guard=sysreg
2. -mstack-protector-guard-reg=sp_el0
3. -mstack-protector-guard-offset=0

to use the system register sp_el0 for the stack canary, enabling the
kernel to have a unique stack canary per task (like a thread, but not
limited to userspace as the kernel can preempt itself).

Address pr/47341 for aarch64.

Fixes: https://github.com/ClangBuiltLinux/linux/issues/289


Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed By: xiangzhangllvm, DavidSpickett, dmgreen

Differential Revision: https://reviews.llvm.org/D100919