[dev.link] cmd/link: fix up ctxt.Textp2 handling in AssignTextSymbolOrder

[dev.link] cmd/link: fix up ctxt.Textp2 handling in AssignTextSymbolOrder

Change the loader method AssignTextSymbolOrder to return a slice of
all reachable textp symbols, since it will be needed in second-phase
DWARF gen.

Change-Id: Iaf16ee9cf0d5266aeb0d3df596e8117263b35d8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/220985
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>