cmd/link: always mark runtime.buildVersion and runtime.modinfo reachable

runtime.buildVersion and runtime.modinfo are referenced in the
.go.buildinfo section, therefore reachable. They should normally
be reachable from the runtime. Just make it explicit, in case.

Change-Id: I60ff3132e0bbb690f4a3cba8bb18735921fbe951
Reviewed-on: https://go-review.googlesource.com/c/go/+/261637


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