[NFC][XCOFF][AIX] Serialize object file writing for each CsectGroup

Summary:

Right now we handle each CsectGroup(ProgramCodeCsects, BSSCsects)
individually when assigning indices, writing symbol table, and
writing section raw data. However, there is already a pattern there,
and we could common up those actions for every CsectGroup. This will
 make adding new CsectGroup(Read Write data, Read only data, TC/TOC,
 mergeable string) easier, and less error prone.

Reviewed by: sfertile, daltenty, DiggerLin

Approved by: daltenty

Differential Revision: https://reviews.llvm.org/D69112