Without this running go vet -compiler=gccgo causes vet to fail.
The vet tool does need to know the compiler, but it is passed in
vetConfig.Compiler.

Change-Id: If857be4f336f6d7c425972fabcf82fae2cdc8a90
Reviewed-on: https://go-review.googlesource.com/113715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>