cmd/objdump: make test independent of inlining

Fixes #19189.

Change-Id: Ice69216c7fc2eaeb3dbbdcd08a8284204c7f52ef
Reviewed-on: https://go-review.googlesource.com/37237


Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>