[dev.link] cmd/link: refactor symbol to section assignment in allocateSections

[dev.link] cmd/link: refactor symbol to section assignment in allocateSections

Create a couple of helper routines to aid in assigning symbols to
sections in dodata's allocateSections, then replace loops over symbol
lists with calls to the helpers, to reduce the amount of duplicate
code.

This patch also decouples gcprog/gcdata generation from
symbol-to-section assignment (previously intertwined), as an aid to
making the code less complicated.

No change in functionality.

Change-Id: If126579486bce458f697e32bad556df453df53e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/228781
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>