[dev.cc] runtime/cgo: fix freebsd build?

Last try and then someone with a FreeBSD has to do it.

TBR=r
CC=golang-codereviews
https://golang.org/cl/171590043

src/runtime/cgo/freebsd.go

@@ -15,5 +15,8 @@ import _ "unsafe"
 //go:linkname _environ environ
 //go:linkname _progname __progname
 
+//go:cgo_export_static environ
+//go:cgo_export_static __progname
+
 var _environ uintptr
 var _progname uintptr