Fixes #10092

This change makes it possible to use gccgo 5 as the GOROOT_BOOTSTRAP
compiler.

Change-Id: Ie3a312781ac1a09ea77f95b5a78c9488d437e0aa
Reviewed-on: https://go-review.googlesource.com/8809
Run-TryBot: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>