go/build: do not consider "android.go" to be android-specific
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 issue 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://codereview.appspot.com/147690043
doc/go1.4.txt: GOOS suffixes require underscores to act as build tags

CC=golang-codereviews
https://codereview.appspot.com/151490043