api: add FreeBSD to go1 API

Now that gri has made go/parser 15% faster, I offer this
change to slow back down cmd/api ~proportionately, adding
FreeBSD to the go1-checked set of platforms.

Really we should have done this earlier. This will prevent us
from breaking FreeBSD compatibility accidentally in the
future.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6279044

src/cmd/api/goapi.go

@@ -57,6 +57,8 @@ var contexts = []*build.Context{
 	{GOOS: "darwin", GOARCH: "amd64"},
 	{GOOS: "windows", GOARCH: "amd64"},
 	{GOOS: "windows", GOARCH: "386"},
+	{GOOS: "freebsd", GOARCH: "amd64"},
+	{GOOS: "freebsd", GOARCH: "386"},
 }
 
 func contextName(c *build.Context) string {