cmd/go: trim objdir, not just workdir, from object files

cmd/go: trim objdir, not just workdir, from object files

Otherwise the new numbered directories like b028/ appear in the objects,
and they can change from run to run.

Fixes #22514.

Change-Id: I8d0cf65f3622e48b2547d5757febe0ee1301e2ed
Reviewed-on: https://go-review.googlesource.com/74791
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>