[AIX][XCOFF] Add support for generating assembly code for one-byte mergable strings

This patch adds support for generating assembly code for one-byte mergeable strings.

Generating assembly code for multi-byte mergeable strings and the `XCOFF` object code for mergeable strings will be supported later.

Reviewers: hubert.reinterpretcast, jasonliu, daltenty, sfertile, DiggerLin, Xiangling_L

Reviewed by: daltenty

Subscribers: wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70310