This enables #cgo directives to contain a os/arch
specification which restricts the definition of
the given option to matching systems.

For example:

#cgo amd64 CFLAGS: -DAMD64=1
#cgo linux CFLAGS: -DLINUX=1
#cgo linux/amd64 CFLAGS: -DLINUX_ON_AMD64=1

R=rsc
CC=golang-dev
https://golang.org/cl/4121048