cmd/go: always link external test packages first when using gccgo
This CL is an amagamation of several fixes Canonical have made on their
fork of the cmd/go tool (packaged as gccgo-go.deb on Ubuntu 14.04+).
Additionally this CL brings gccgoToolchain.ldi() up to date with the version
that will ship in gccgo-5.0. As gccgo is most likely to be used with its
own version of the go tool that it supples it makes good sense that the libgo
version should dictate the contents of gccgotoolchain.ld()
Please see https://codereview.appspot.com/
222890043/ for more details on the
issues fixed.
Change-Id: Icf7deb43f8e80b424757f1673e6bca7a0aa2a1ac
Reviewed-on: https://go-review.googlesource.com/8250
Reviewed-by: Ian Lance Taylor <iant@golang.org>