cmd/internal/obj: build ctxt.Text during Sym init

cmd/internal/obj: build ctxt.Text during Sym init

Instead of constructing ctxt.Text in Flushplist,
which will be called concurrently,
do it in InitTextSym, which must be called serially.
This allows us to avoid a mutex for ctxt.Text,
and preserves the existing ordering of functions
for debug output.

Passes toolstash-check.

Updates #15756

Change-Id: I6322b4da24f9f0db7ba25e5b1b50e8d3be2deb37
Reviewed-on: https://go-review.googlesource.com/40502
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>