cmd/go: don't give an error for an attempt to recreate a symlink
cmd/go: don't give an error for an attempt to recreate a symlink

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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,golang/go,golang/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,akutz/go,josharian/go,nawawi/go,josharian/go,CAFxX/go,CAFxX/go,CAFxX/go,josharian/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,golang/go,nawawi/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,akutz/go,josharian/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,akutz/go,josharian/go,golang/go,cloudflare/go,CAFxX/go