cmd/go: for gccgo expect one fewer file in TestScript/list_swigcxx

cmd/go: for gccgo expect one fewer file in TestScript/list_swigcxx

One of the files in CompileGoFiles is actually _cgo_import.go, but
that file is only generated for gc, not for gccgo.

Change-Id: I87bb55552e1409cc57da8f35a32b37ce4a3df60c
Reviewed-on: https://go-review.googlesource.com/c/go/+/390895
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>