[XCOFF][AIX] Support basic relocation type on AIX

Summary:

This patch intends to support three most common relocation type
on AIX: R_POS, R_TOC, R_RBR.
These three relocation type will be needed for object file generation
on AIX for small code model.
We will have follow up patches to bring relocation support for
large code model on AIX.

Reviewers: hubert.reinterpretcast, daltenty, DiggerLin

Differential Revision: https://reviews.llvm.org/D72027