cmd/go: fix testsuite for gccgo

A number of cmd/go tests can never work with gccgo, for various
different reasons. Skip those tests when using gccgo. Adjust some
other tests to pass when using gccgo. Adjust one test to not skip when
using gccgo, since it does work.

Change-Id: I33b09558581a1e304416cf1c05a96f9526abba0e
Reviewed-on: https://go-review.googlesource.com/110915
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>