misc/cgo/testcshared: reapply CL 451816

misc/cgo/testcshared: reapply CL 451816

I accidentally reverted its edits with a bad cherry-pick in CL 452457.

This should re-fix the windows-.*-newcc builders that regressed at
that change.

Updates #47257.
Updates #35006.
Updates #53540.

Change-Id: I5818416af7c4c8c1593c36aa0198331b42b6c7d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/452675
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>