Replace remaining uses with loader.Syms. Reduces some memory
usage.

Change-Id: I6f295b42b8cd734c6c18f08c61a5473506675075
Reviewed-on: https://go-review.googlesource.com/c/go/+/229992


Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>