cmd/go: be more careful when linking a test exe with gccgo
Previously, we ended up passing two compiled objects for the package
being tested when linking the test executable. Somewhat by luck, this
worked most of the time but occasionally it did not. This changes the
linking code to not pass two objects for the same ImportPath and to
always pass the object for the test version of the package and removes
some unecessary nil checks.
Change-Id: I7bbd3fc708f14672ee2cc6aed3397421fceb8a38
Reviewed-on: https://go-review.googlesource.com/1840 Reviewed-by: Ian Lance Taylor <iant@golang.org>