[NFC][XCOFF] Refactor Csect creation into TargetLoweringObjectFile

Summary:
We create a number of standard types of control sections in multiple places for
things like the function descriptors, external references and the TOC anchor
among others, so it is possible for  their properties to be defined
inconsistently in different places. This refactor moves their creation and
properties into functions in the TargetLoweringObjectFile class hierarchy, where
functions for retrieving various special types of sections typically seem
to reside.

Note: There is one case in PPCISelLowering which is specific to function entry
points which we don't address since we don't have access to the TLOF there.

Reviewers: DiggerLin, jasonliu, hubert.reinterpretcast

Reviewed By: jasonliu, hubert.reinterpretcast

Subscribers: wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72347