When used with the go tool, gccgo will normally generate archive files.
This change teaches the gccgoimporter package how to read the export
data from an archive.

This is needed by, for example, cmd/vet, when typechecking packages.

Change-Id: I21267949a7808cd81c0042af425c774a4ff7d82f
Reviewed-on: https://go-review.googlesource.com/119895
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>