go/build, cmd/go: don't expect gccgo to have GOROOT packages

When using gccgo the standard library sources are not available in
GOROOT. Don't expect them to be there. In the gccgo build, use a set
of standard library packages generated at build time.

Change-Id: Id133022604d9b7e778e73e8512f9080c61462fba
Reviewed-on: https://go-review.googlesource.com/111595


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>