When building for gccgo cmd/go uses symlinks for import maps.
In some cases, such as TestVendorTest, it generates the same symlink
multiple times. Don't give an error when this happens.

Change-Id: Iecc154ea1ac53d7c5427b36795881909c5cac7e3
Reviewed-on: https://go-review.googlesource.com/111636
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>