[dev.link] cmd/link: add symbols to Textp after deadcode pass

[dev.link] cmd/link: add symbols to Textp after deadcode pass

Currently we add all text symbols to ctxt.Textp at load time,
then the deadcode pass filters out unreachable ones. This CL
delays adding symbols to ctxt.Textp to the end of the deadcode
pass, where we only add reachable ones.

Change-Id: Ie83b2958f915c5aaa004b8c5ed1f1bc275f4d1db
Reviewed-on: https://go-review.googlesource.com/c/go/+/197257
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>