The go/build package already recognizes
system-specific file names like

        mycode_darwin.go
        mycode_darwin_386.go
        mycode_386.s

However, it is also common to write files that
apply to multiple architectures, so a recent CL added
to go/build the ability to process comments
listing a set of conditions for building.  For example:

        // +build darwin freebsd openbsd/386

says that this file should be compiled only on
OS X, FreeBSD, or 32-bit x86 OpenBSD systems.

These conventions are not yet documented
(hence this long CL description).

This CL adds build comments to the multi-system
files in the core library, a step toward making it
possible to use go/build to build them.

With this change go/build can handle crypto/rand,
exec, net, path/filepath, os/user, and time.

os and syscall need additional adjustments.

R=golang-dev, r, gri, r, gustavo
CC=golang-dev
https://golang.org/cl/5011046