cmd/internal/obj: refactor ATEXT symbol initialization

cmd/internal/obj: refactor ATEXT symbol initialization

This makes the core Flushplist loop clearer.

We may also want to move the Sym initialization
much earlier in the compiler (see discussion on
CL 40254), for which this paves the way.

While we're here, eliminate package log in favor of ctxt.Diag.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ieaf848d196764a5aa82578b689af7bc6638c385a
Reviewed-on: https://go-review.googlesource.com/40313
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>