debug/elf: add FuzzReader

Add a fuzzing test for debug/elf similar to the one added for debug/pe
in https://go.dev/cl/780360.

Change-Id: If6e19a95f65f1ace2eb768a5184d700a932f6378
Reviewed-on: https://go-review.googlesource.com/c/go/+/781180
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>