Rebuild cmd/objdump once instead of twice.
Speeds up standalone 'go test cmd/objdump' on my
machine from ~1.4s to ~1s.

Updates #17751

Change-Id: I15fd79cf18c310f892bc28a9e9ca47ee010c989a
Reviewed-on: https://go-review.googlesource.com/32673
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>