We're going to start building cmd/cgo as part of the bootstrap,
and with it debug/elf, so the copy here needs to work with Go 1.4.
It does except for the use of the new io.SeekStart etc constants,
so remove that use.

Change-Id: Ib7fcf46e1e9060f96d2bacaaf349c9b0df347550
Reviewed-on: https://go-review.googlesource.com/68337
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>