cmd/go: skip TestScript/gccgo_link_ldflags on aix/ppc64

cmd/go: skip TestScript/gccgo_link_ldflags on aix/ppc64

The gccgo on the builder is not updated to support runtime/cgo

Updates #60306

Change-Id: If0fb1ccdf589cc9741f6a065bacfa4f06e64ec15
Reviewed-on: https://go-review.googlesource.com/c/go/+/501435
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>