Add AIX assembler support

Summary:
A skeleton of AIX toolchain and system linker support has been introduced in D68340, and this is a follow on patch to it.
This patch adds support to system assembler invocation to the AIX toolchain.

Reviewers: daltenty, hubert.reinterpretcast, jasonliu, Xiangling_L, dlj

Reviewed By: daltenty, hubert.reinterpretcast

Subscribers: wuzish, nemanjai, kbarton, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69620