The gccgo compiler does not have the standard packages available, so it
can not check for violations of internal references.

Also, the gccgo compiler can not read runtime/internal/sys/zversion.go;
in fact, the file does not even exist for gccgo.

Change-Id: Ibadf16b371621ad1b87b6e858c5eb233913e179d
Reviewed-on: https://go-review.googlesource.com/33295
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>