cmd/go: skip tests that do not work with gccgo

These tests fail for various reasons, most commonly because gccgo
doesn't really have GOROOT, so things like `go build errors` fail.

Change-Id: I4c0cb95a661be552725b10675dd4cca7f132323a
Reviewed-on: https://go-review.googlesource.com/45696
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed