A file name must have a non-empty underscore-separated
prefix before its suffix matches GOOS. This is what the
documentation already said but is not what the code did.

Fixes #8838.

This needs to be called out in the release notes.
The he single affected file
        code.google.com/p/go.text/collate/tools/colcmp/darwin.go
could use a renaming but works because it has a build tag inside.

LGTM=adg, rsc
R=golang-codereviews, adg, rsc
CC=golang-codereviews
https://golang.org/cl/147690043