[AIX][XCOFF]Implement mergeable const

SUMMARY:
In this patch, we map mergeable const objects to the read-only section in the same manner as const objects that are not mergeable.

Reviewers: hubert.reinterpretcast,jasonliu
Subscribers: wuzish, nemanjai, hiraditya

Differential Revision: https://reviews.llvm.org/D71551