go/build: support Import of local import path in standard library for gccgo

It's possible for a local import path to refer to a standard library
package. This was not being correctly handled for gccgo. When using
gccgo, change the code to permit the existing lexical test, and to
accept a missing directory for a standard package found via a local
impor path.

Change-Id: Ia9829e55c0ff62e7d1f01a1d6dc9fcff521501ca
Reviewed-on: https://go-review.googlesource.com/137439


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>