This version of the file name honors the -trimprefix flag,
which strips off variable parts like $WORK or $PWD.
The TestCgoConsistentResults test now passes.

Change-Id: If93980b054f9b13582dd314f9d082c26eaac4f41
Reviewed-on: https://go-review.googlesource.com/22444Reviewed-by: Robert Griesemer <gri@golang.org>