[dev.link] cmd/link: fix buglet in setupdynexp
[dev.link] cmd/link: fix buglet in setupdynexp

This should restore deterministic order of dynexp, and fix
Solaris build.

Change-Id: Icb796babaa3238bff90fd8255ee9f023f2306c26
Reviewed-on: https://go-review.googlesource.com/c/go/+/225538
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go