cmd/go: don't check standard packages when using gccgo

cmd/go: don't check standard packages when using gccgo

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>