This is in preparation of delaying loading symbol content to
after dead code elimination.

Change-Id: Ia4ce82d9a42cc1a76c82f6d0eb9a54b0325ee3ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/197061
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>