go/internal/gccgoimporter: fix test when using gccgo 4.7

TestInstallationImporter checks that it can read the export data for a
list of known standard library packages. It was failing on the SmartOS
builder which has GCC 4.7 installed. Skip packages that did not exist
in GCC 4.7. Most packages are still there and the missing packages are
fairly simple, so this doesn't really affect test quality.

Updates #29006

Change-Id: If7ae6f83d51d40168a9692acb0b99c9bf21f2a4d
Reviewed-on: https://go-review.googlesource.com/c/152077


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