cmd/internal/obj: refactor code to separate content-addressable symbols by section

cmd/internal/obj: refactor code to separate content-addressable symbols by section

The goal of this change is to improve the documentation
and make it easier to keep Link.NumberSyms and writer.contentHash aligned.
No functional changes.

A subsequent change will add conditions to contentHashSection.

Change-Id: I0a274f6974459d34d5a8553081f33ea4cd87f248
Reviewed-on: https://go-review.googlesource.com/c/go/+/352669
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>