cmd/link: build dynexp symbol list directly

Currently, setCgoAttr populates the cgo_export_{static,dynamic} maps
with symbol names of exported symbols, which are then re-looked-up by
deadcode and setupdynexp, which in turn puts the re-looked-up symbols
in ctxt.dynexp. setCgoAttr already looked up the Syms, so simplify all
of this by making setCgoAttr populate ctxt.dynexp directly and
eliminating the cgo_export_{static,dynamic} maps. Recording Syms
directly also sets us up to use correct symbol versions for these
exports, rather than just assuming version 0 for all lookups.

Since setupdynexp doesn't really do any "setting up" of dynexp any
more with this change, we fold the remaining logic from setupdynexp
directly into addexport, where it has better context anyway. This also
eliminates a sorting step, since we no longer do a non-deterministic
map iteration to build the dynexp slice.

For #40724.

Change-Id: I3e1a65165268da8c2bf50d7485f2624133433260
Reviewed-on: https://go-review.googlesource.com/c/go/+/309340


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>