debug/gosym: correct comments for Table.{Files,Objs}

The fields aren't too useful for Go 1.2 and later, but they aren't
actually nil.

Fixes #38754

Change-Id: Ia13a224f623697a00dea8ba0225633e1b9308c9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/230940


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>