[XCOFF][AIX] Use 'L..' instead of '.L' for getPrivateGlobalPrefix in DataLayout

Summary:
D80831 changed part of the prefix usage for AIX.
But there are other places getting prefix from DataLayout.
This patch intends to make prefix usage consistent on AIX.

Reviewed by: hubert.reinterpretcast, daltenty

Differential Revision: https://reviews.llvm.org/D81270