[XCOFF][AIX] Handle TOC entries that could not be reached by positive range in small code model

Summary:
In small code model, AIX assembler could not deal with labels that
could not be reached within the [-0x8000, 0x8000) range from TOC base.
So when generating the assembly, we would need to help the assembler
by subtracting an offset from the label to keep the actual value
within [-0x8000, 0x8000).

Reviewed By: hubert.reinterpretcast, Xiangling_L

Differential Revision: https://reviews.llvm.org/D86879