[AIX] Enable stackprotect feature

AIX use `__ssp_canary_word` instead of `__stack_chk_guard`.
This patch update the target hook to use correct symbol,
so that the basic stackprotect feature can work.

The traceback will be handled in follow up patch.

Reviewed By: #powerpc, shchenz

Differential Revision: https://reviews.llvm.org/D103100