cmd/go: re-enable a couple of tests of gccgo

Port https://golang.org/cl/120375 over to the gofrontend repo so that
it gets more reliable testing.

Updates golang/go#22472

Change-Id: I314396e49934374a143a025a477f01449ac1812b
Reviewed-on: https://go-review.googlesource.com/120395
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
1 file changed